Derivative and Hedging Activities - Cash Flow Hedges Components of AOCI (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, after tax, beginning of period				$ (23)
Losses reclassified to income, after tax	[1]	$ 8		18
Losses reclassified to income, after tax	[1],[2]		$ 7		$ 17
AOCI, after tax, end of period		(5)		(5)
Cash flow hedging
AOCI Attributable to Parent, Net of Tax [Roll Forward]
AOCI, before tax, beginning of period		(70)	(104)	(80)	(114)
AOCI, after tax, beginning of period		(13)	(47)	(23)	(57)
Losses reclassified to income, before tax		8		18
Losses reclassified to income, after tax		8		18
Losses reclassified to income, before tax			7		17
Losses reclassified to income, after tax			7		17
AOCI, before tax, end of period		(62)	(97)	(62)	(97)
AOCI, after tax, end of period		$ (5)	$ (40)	$ (5)	$ (40)

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

[2] Financial information for 2018 has been recast to reflect the retrospective application of the successful efforts method of accounting. See Note 1.