diff --git a/google/auth/_helpers.py b/google/auth/_helpers.py
index 9fe2caa..92c6c83 100644
--- a/google/auth/_helpers.py
+++ b/google/auth/_helpers.py
@@ -22,9 +22,27 @@
 
 
 def copy_docstring(source_class):
-    """Decorator that copies a method's docstring from another class."""
+    """Decorator that copies a method's docstring from another class.
+
+    Args:
+        source_class (type): The class that has the documented method.
+
+    Returns:
+        Callable: A decorator that will copy the docstring of the same
+            named method in the source class to the decorated method.
+    """
     def decorator(method):
-        """Decorator implementation."""
+        """Decorator implementation.
+
+        Args:
+            method: The method to copy the docstring to.
+
+        Returns:
+            Callable: the same method passed in with an updated docstring.
+
+        Raises:
+            ValueError: if the method already has a docstring.
+        """
         if method.__doc__:
             raise ValueError('Method already has a docstring.')