Derivative Financial Instruments Hedged Items in Fair Value Hedging Relationships (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Hedged Items in Fair Value Hedging Relationships [Line Items]
Document Period End Date	Sep. 30, 2019
Fair Value Hedging [Member] | Long-term Debt [Member]
Hedged Items in Fair Value Hedging Relationships [Line Items]
Carrying Amount of Assets	$ 11,295,000,000	$ 8,411,000,000
Fair Value Hedging [Member] | Long-term Debt [Member] | Designated as Hedging Instrument [Member]
Hedged Items in Fair Value Hedging Relationships [Line Items]
Hedged Asset, Cumulative Basis Adjustment	180,000,000	(10,000,000)
Fair Value Hedging [Member] | Long-term Debt [Member] | Not Designated as Hedging Instrument [Member]
Hedged Items in Fair Value Hedging Relationships [Line Items]
Hedged Asset, Cumulative Basis Adjustment	$ (28,000,000)	(120,000,000)
Fair Value Hedging [Member] | Brokered Time Deposits [Member]
Hedged Items in Fair Value Hedging Relationships [Line Items]
Carrying Amount of Assets		29,000,000
Fair Value Hedging [Member] | Brokered Time Deposits [Member] | Designated as Hedging Instrument [Member]
Hedged Items in Fair Value Hedging Relationships [Line Items]
Hedged Asset, Cumulative Basis Adjustment		0
Fair Value Hedging [Member] | Brokered Time Deposits [Member] | Not Designated as Hedging Instrument [Member]
Hedged Items in Fair Value Hedging Relationships [Line Items]
Hedged Asset, Cumulative Basis Adjustment		$ 0