Derivative and Hedging Activities - Cash Flow Hedges Components of AOCI Table (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Net of Tax Beginning of Period			$ (96)
Other Comprehensive Income (Loss), Net Change in Fair Value, Net of Tax	$ 0	$ (15)	4	$ (19)
Other Comprehensive Income (Loss), Losses Reclassified to Income, Net of Tax	7	10	17	14
Accumulated Other Comprehensive Income (Loss), Net of Tax End of Period	(75)		(75)
Cash Flow Hedging [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Before Tax, Period Start	(139)	(156)	(153)	(160)
Accumulated Other Comprehensive Income (Loss), Net of Tax Beginning of Period	(82)	(99)	(96)	(99)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax	0	(13)	4	(20)
Other Comprehensive Income (Loss), Losses Reclassified to Income, Before Tax	7	6	17	17
Other Comprehensive Income (Loss), Losses Reclassified to Income, Net of Tax	7	10	17	14
Accumulated Other Comprehensive Income (Loss), Before Tax, Period End	(132)	(163)	(132)	(163)
Accumulated Other Comprehensive Income (Loss), Net of Tax End of Period	$ (75)	$ (104)	$ (75)	$ (104)