Derivative and Hedging Activities - Cash Flow Hedges Components of AOCI Table (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 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	$ (4)	4	$ (23)
Other Comprehensive Income (Loss), Losses Reclassified to Income, Net of Tax	8	7	25	21
Accumulated Other Comprehensive Income (Loss), Net of Tax End of Period	(67)		(67)
Cash Flow Hedging [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Before Tax, Beginning of Periodt	(132)	(163)	(153)	(160)
Accumulated Other Comprehensive Income (Loss), Net of Tax Beginning of Period	(75)	(104)	(96)	(99)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Before Tax	0	(4)	4	(23)
Other Comprehensive Income (Loss), Losses Reclassified to Income, Before Tax	8	7	25	23
Other Comprehensive Income (Loss), Losses Reclassified to Income, Net of Tax	8	7	25	21
Accumulated Other Comprehensive Income (Loss), Before Tax, End of Period	(124)	(160)	(124)	(160)
Accumulated Other Comprehensive Income (Loss), Net of Tax End of Period	$ (67)	$ (101)	$ (67)	$ (101)