Derivative and Hedging Activities - Cash Flow Hedges Components of AOCI Table (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, after tax, beginning of period				$ (57)
Net change in fair value, after tax	[1]	$ 0	$ 0	0	$ 4
Losses reclassified to income, after tax	[1]	7	7	17	17
AOCI, after tax, end of period		(40)		(40)
Cash flow hedging [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, before tax, beginning of period		(104)	(139)	(114)	(153)
AOCI, after tax, beginning of period		(47)	(82)	(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		7	7	17	17
Losses reclassified to income, after tax		7	7	17	17
AOCI, before tax, end of period		(97)	(132)	(97)	(132)
AOCI, after tax, end of period		$ (40)	$ (75)	$ (40)	$ (75)

[1] Deferred tax activity incurred in other comprehensive income was offset by a valuation allowance.