Derivatives and Hedging Activities - After-Tax Change in AOCI Resulting from Cash Flow Hedges (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
December 31, 2016		$ (14)
Reclassification of tax effects in AOCI resulting from the new federal corporate income tax rate	$ 141	141
December 31, 2017	$ (86)	(86)
Unrealized Gains (Losses) on Derivative Financial Instruments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
2017 Hedging Activity		(48)
Reclassification of Gains to Net Income		(10)
Reclassification of tax effects in AOCI resulting from the new federal corporate income tax rate		$ (14)