Financial Derivative Instruments - Carrying Amount of Fair Value Hedges (Details) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Deferred Gain (Loss) on Discontinuation of Interest Rate Fair Value Hedge		$ 20	$ 21
Long-term Debt [Member] | Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Amount of Hedged Item		783	796
Cumulative fair value adjustment [Member] | Long-term Debt [Member] | Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Amount of Hedged Item	[1]	$ 3	$ 17

[1] At September 30, 2018 and 2017, these amounts include the cumulative amount of fair value hedging adjustments remaining for which hedge accounting has been discontinued of $20 million and $21 million, respectively.