diff --git a/stix2/__init__.py b/stix2/__init__.py
index d022302..9b7e1c3 100644
--- a/stix2/__init__.py
+++ b/stix2/__init__.py
@@ -21,17 +21,29 @@ def format_datetime(dt):
 class _STIXBase(collections.Mapping):
     """Base class for STIX object types"""
 
-    def _check_kwargs(self, **kwargs):
-        class_name = self.__class__.__name__
+    @classmethod
+    def _make_id(cls):
+        return cls._type + "--" + str(uuid.uuid4())
+
+    @classmethod
+    def _check_kwargs(cls, **kwargs):
+        class_name = cls.__name__
 
         # Ensure that, if provided, the 'type' kwarg is correct.
-        required_type = self.__class__._type
+        required_type = cls._type
         if not kwargs.get('type'):
             kwargs['type'] = required_type
         if kwargs['type'] != required_type:
             msg = "{0} must have type='{1}'."
             raise ValueError(msg.format(class_name, required_type))
 
+        id_prefix = cls._type + "--"
+        if not kwargs.get('id'):
+            kwargs['id'] = cls._make_id()
+        if not kwargs['id'].startswith(id_prefix):
+            msg = "{0} id values must begin with '{1}'."
+            raise ValueError(msg.format(class_name, id_prefix))
+
         return kwargs
 
     def __init__(self, **kwargs):
@@ -84,11 +96,6 @@ class Bundle(_STIXBase):
         # TODO: remove once we check all the fields in the right order
         kwargs = self._check_kwargs(**kwargs)
 
-        if not kwargs.get('id'):
-            kwargs['id'] = 'bundle--' + str(uuid.uuid4())
-        if not kwargs['id'].startswith('bundle--'):
-            raise ValueError("Bundle id values must begin with 'bundle--'.")
-
         if not kwargs.get('spec_version'):
             kwargs['spec_version'] = '2.0'
         if kwargs['spec_version'] != '2.0':
@@ -142,11 +149,6 @@ class Indicator(_STIXBase):
         # TODO: remove once we check all the fields in the right order
         kwargs = self._check_kwargs(**kwargs)
 
-        if not kwargs.get('id'):
-            kwargs['id'] = 'indicator--' + str(uuid.uuid4())
-        if not kwargs['id'].startswith('indicator--'):
-            raise ValueError("Indicator id values must begin with 'indicator--'.")
-
         if not kwargs.get('labels'):
             raise ValueError("Missing required field for Indicator: 'labels'.")
 
@@ -202,16 +204,6 @@ class Malware(_STIXBase):
         # TODO: remove once we check all the fields in the right order
         kwargs = self._check_kwargs(**kwargs)
 
-        if not kwargs.get('type'):
-            kwargs['type'] = 'malware'
-        if kwargs['type'] != 'malware':
-            raise ValueError("Malware must have type='malware'.")
-
-        if not kwargs.get('id'):
-            kwargs['id'] = 'malware--' + str(uuid.uuid4())
-        if not kwargs['id'].startswith('malware--'):
-            raise ValueError("Malware id values must begin with 'malware--'.")
-
         if not kwargs.get('labels'):
             raise ValueError("Missing required field for Malware: 'labels'.")
 
@@ -274,11 +266,6 @@ class Relationship(_STIXBase):
         # if we won't need it?
         now = datetime.datetime.now(tz=pytz.UTC)
 
-        if not kwargs.get('type'):
-            kwargs['type'] = 'relationship'
-        if kwargs['type'] != 'relationship':
-            raise ValueError("Relationship must have type='relationship'.")
-
         if not kwargs.get('id'):
             kwargs['id'] = 'relationship--' + str(uuid.uuid4())
         if not kwargs['id'].startswith('relationship--'):
@@ -301,6 +288,7 @@ class Relationship(_STIXBase):
             'created': kwargs.get('created', now),
             'modified': kwargs.get('modified', now),
             'relationship_type': kwargs['relationship_type'],
+            'target_ref': kwargs['target_ref'],
         })
         super(Relationship, self).__init__(**kwargs)