Financial Derivative Instruments - Carrying Amount of Fair Value Hedges (Details) - USD ($)		Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Deferred Gain (Loss) on Discontinuation of Interest Rate Fair Value Hedge		$ 20,000,000	$ 21,000,000
Designated as Hedging Instrument | Long-term Debt [Member]
Derivative [Line Items]
Derivative, Amount of Hedged Item		791,000,000	791,000,000
cumulative fair value adjustment [Member] | Designated as Hedging Instrument | Long-term Debt [Member]
Derivative [Line Items]
Derivative, Amount of Hedged Item	[1]	$ 11,000,000	$ 12,000,000

[1] At December 31, 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.