Derivative and Hedging Activities - Cash Flow Hedges Components of AOCI Table (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, after tax, beginning of period	$ (57)
Net change in fair value, after tax	0	$ 4
Losses reclassified to income, after tax	10	10
AOCI, after tax, end of period	(47)
Cash flow hedging [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, before tax, beginning of period	(114)	(153)
AOCI, after tax, beginning of period	(57)	(96)
Net change in fair value, before tax	0	4
Net change in fair value, after tax	0	4
Losses reclassified to income, before tax	10	10
Losses reclassified to income, after tax	10	10
AOCI, before tax, end of period	(104)	(139)
AOCI, after tax, end of period	$ (47)	$ (82)