Derivative Instruments (Change in Unrealized (Losses) Gains in Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Net unrealized gains (losses) on cash flow hedges, net of taxes, beginning of period	$ 650
Net unrealized gains on cash flow hedges, net of taxes, end of period	789
Unrealized Gain (Loss) on Forward Exchange Contracts
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Net unrealized gains (losses) on cash flow hedges, net of taxes, beginning of period	2	$ (1)
Change in fair value, net of tax	6	4
Reclassification into earnings, net of tax	(1)	(1)
Net unrealized gains on cash flow hedges, net of taxes, end of period	$ 7	$ 2