Derivative and Hedging Activities - Cash Flow Hedges Components of AOCI Table (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 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]	8	8	25	25
AOCI, after tax, end of period		(32)		(32)
Cash flow hedging [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, before tax, beginning of period		(97)	(132)	(114)	(153)
AOCI, after tax, beginning of period		(40)	(75)	(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		8	8	25	25
Losses reclassified to income, after tax		8	8	25	25
AOCI, before tax, end of period		(89)	(124)	(89)	(124)
AOCI, after tax, end of period		$ (32)	$ (67)	$ (32)	$ (67)

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