Derivative Instruments and Hedging Activities (Narrative) (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($) derivative_instrument	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) derivative_instrument	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, notional amount	$ 19,073,000,000		$ 19,073,000,000		$ 17,877,000,000
Amortization of cumulative debt basis adjustments	3,000,000
Redesignated swaps, unamortized basis adjustments	7,000,000		7,000,000
Decrease in cash flow hedge portfolio	12,000,000
Other liabilities	44,000,000		44,000,000		38,000,000
Cash collateral pledged for derivative liabilities	61,000,000		61,000,000
Additional collateral that would be required to be pledge if credit rating was downgraded by one notch	2,000,000		2,000,000
Other liabilities
Derivative Instruments, Gain (Loss) [Line Items]
Other liabilities	295,000,000		295,000,000		418,000,000
Derivatives designated as hedges
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, notional amount	4,833,000,000		4,833,000,000		4,033,000,000
Decrease in cash flow hedge portfolio	4,000,000	$ (10,000,000)	9,000,000	$ (102,000,000)
Derivatives designated as hedges | Other liabilities
Derivative Instruments, Gain (Loss) [Line Items]
Other liabilities	$ 0		$ 0		0
Fair Value Hedging | Derivatives designated as hedges | Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Number of interest rate derivatives remaining | derivative_instrument	1		1
Derivative, notional amount	$ 500,000,000		$ 500,000,000		500,000,000
Increase (decrease) in fair value hedges	12,000,000
Unamortized debt basis adjustments	5,000,000		5,000,000
Fair Value Hedging | Derivatives designated as hedges | Asset hedges: Pay-fixed interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, notional amount	383,000,000		383,000,000		383,000,000
Increase (decrease) in fair value hedges	(25,000,000)
Decrease in cash flow hedge portfolio	0	0	0	0
Fair Value Hedging | Derivatives designated as hedges | Other liabilities | Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Other liabilities	0		0		0
Fair Value Hedging | Derivatives designated as hedges | Other liabilities | Asset hedges: Pay-fixed interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Other liabilities	0		0		0
Cash Flow Hedging | Derivatives designated as hedges | Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, notional amount	3,950,000,000		3,950,000,000		3,150,000,000
Decrease in cash flow hedge portfolio	4,000,000	$ (10,000,000)	9,000,000	$ (102,000,000)
Cash Flow Hedging | Derivatives designated as hedges | Other liabilities | Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Other liabilities	$ 0		$ 0		$ 0