Derivative Financial Instruments - Net Gain (Loss) Recognized on Designated Instruments (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Net interest income	$ 152,000,000	$ 146,000,000	$ 138,000,000	$ 126,000,000	$ 152,000,000	$ 124,000,000	$ 115,000,000	$ 106,000,000	$ 562,000,000	$ 497,000,000	$ 390,000,000
Fair Value Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain on hedges										0
Cash Flow Hedges
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain reclassified from AOCI into income										30,000,000	$ 5,000,000
Designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Gain on hedges									0	30,000,000
Interest rate contract | Cash Flow Hedges
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain reclassified from AOCI into income									$ 0	$ 30,000,000