Income Taxes - Rollforward of Gross Consolidated Liability for Unrecognized Income Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits, beginning of year	$ 383	$ 266	$ 270
Tax positions related to current year:
Additions	104	63	37
Reductions	0	0	0
Tax positions related to prior years:
Additions	128	91	10
Reductions	(54)	(31)	(33)
Settlements	(13)	(1)	(5)
Lapses in statutes of limitations	(20)	(5)	(13)
Unrecognized tax benefits, end of year	$ 528	$ 383	$ 266