Derivative and Hedging Activities - Cash Flow Hedges Components of AOCI Table (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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), Unrealized Gain (Loss) on Derivatives Arising During Period, Before Tax			$ 32
Other Comprehensive Income (Loss), Net Change in Fair Value, Net of Tax	5	$ (13)	20
Other Comprehensive Income (Loss), Losses Reclassified to Income, Net of Tax	34	16	24
Accumulated Other Comprehensive Income (Loss), Net of Tax End of Period	(57)	(96)
Cash Flow Hedging [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Before Tax, Beginning of Periodt	(153)	(160)	(231)
Accumulated Other Comprehensive Income (Loss), Net of Tax Beginning of Period	(96)	(99)	(143)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Before Tax	5	(27)
Other Comprehensive Income (Loss), Losses Reclassified to Income, Before Tax	34	34	39
Other Comprehensive Income (Loss), Losses Reclassified to Income, Net of Tax	34	16	24
Accumulated Other Comprehensive Income (Loss), Before Tax, End of Period	(114)	(153)	(160)
Accumulated Other Comprehensive Income (Loss), Net of Tax End of Period	$ (57)	$ (96)	$ (99)