Basis of Presentation (Details)	3 Months Ended
	Mar. 31, 2013 entity
Principles of Consolidation
Number of variable interest entities	38
Joint ventures | Consolidated variable interest entities
Principles of Consolidation
Number of variable interest entities	29
Joint ventures | Voting Interest Model Entities
Principles of Consolidation
Number of variable interest entities	9
Joint ventures | Voting Interest Model Entities | Minimum
Principles of Consolidation
Ownership percentage	20.00%
Joint ventures | Voting Interest Model Entities | Maximum
Principles of Consolidation
Ownership percentage	50.00%