Segment Information - Schedule of Information Concerning Principal Geographic Areas (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Information concerning principal geographic areas
Total revenues	$ 2,943	$ 2,936	$ 2,928	$ 2,777	$ 2,964	$ 2,920	$ 2,872	$ 2,695	$ 11,584	$ 11,451	$ 11,152
Long-Lived Assets	21,118				21,716				21,118	21,716	22,296
United States
Information concerning principal geographic areas
Total revenues									9,890	9,795	9,428
Long-Lived Assets	18,846				19,400				18,846	19,400	19,708
International
Information concerning principal geographic areas
Total revenues									1,694	1,656	1,724
Long-Lived Assets	$ 2,272				$ 2,316				$ 2,272	$ 2,316	$ 2,588