Financial Derivative Instruments - Carrying Amount of Fair Value Hedges (Details) - USD ($)		Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Deferred Gain (Loss) on Discontinuation of Interest Rate Fair Value Hedge		$ 20,000,000	$ 21,000,000
Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Amount of Hedged Item		795,000,000	784,000,000
Short-term Debt | Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Amount of Hedged Item		300,000,000	0
Long-term Debt | Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Amount of Hedged Item		495,000,000	784,000,000
Cumulative fair value adjustment [Member] | Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Amount of Hedged Item	[1]	15,000,000	5,000,000
Cumulative fair value adjustment [Member] | Short-term Debt | Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Amount of Hedged Item	[1]	2,000,000	0
Cumulative fair value adjustment [Member] | Long-term Debt | Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Amount of Hedged Item	[1]	$ 13,000,000	$ 5,000,000

[1] At March 31, 2019 and 2018, 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.