Derivatives and Hedging Activities (Net Gains (Losses) on Fair Value Hedging Relationships) (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Interest Income, Federal Home Loan Bank Advances		$ 659	$ 537	$ 1,315	$ 992
Interest Expense, Other Long-term Debt		(450)	(392)	(916)	(712)
Interest rate swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net gains on derivatives and hedged items in fair value hedging relationships			15		34
Interest Income [Member] | Advances [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Interest Income, Federal Home Loan Bank Advances		659		1,315
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge		483		758
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments		(487)		(758)
GainLossonFairValueHedgesRecognizedinNetInterestIncomeBeforePriceAlignmentInterest		(4)		0
Net Interest Settlements	[1],[2]	13		33
Amortization Accretion of Active Hedging Relationships		(6)		(12)
Other Effects from Fair Value Hedging Relationships		1		1
Gain (Loss) on Fair Value Hedges Recognized in Net Interest Income		4		22
Interest Income [Member] | Designated as Hedging Instrument [Member] | Advances [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge	[3],[4]		0		0
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	[4]		(1)		(34)
Gain (Loss) on Fair Value Hedges Recognized in Net Interest Income	[4]		(1)		(34)
Interest Expense [Member] | Unsecured Debt [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Interest Expense, Other Long-term Debt		(450)		(916)
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge		(80)		(169)
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments		79		164
GainLossonFairValueHedgesRecognizedinNetInterestIncomeBeforePriceAlignmentInterest		(1)		(5)
Net Interest Settlements	[1],[2]	(12)		(30)
Amortization Accretion of Active Hedging Relationships		0		0
Other Effects from Fair Value Hedging Relationships		0		0
Gain (Loss) on Fair Value Hedges Recognized in Net Interest Income		$ (13)		$ (35)
Interest Expense [Member] | Designated as Hedging Instrument [Member] | Unsecured Debt [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge	[3],[4]		0		0
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	[4]		(15)		(11)
Gain (Loss) on Fair Value Hedges Recognized in Net Interest Income	[4]		(15)		(11)
Interest Expense [Member] | Designated as Hedging Instrument [Member] | Short-term Debt [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge	[3],[4]		0		0
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	[4]		(1)		(2)
Gain (Loss) on Fair Value Hedges Recognized in Net Interest Income	[4]		(1)		(2)
Gain (Loss) on Derivative Instruments [Member] | Designated as Hedging Instrument [Member] | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge	[4]		(74)		(261)
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	[4]		89		295
Net gains on derivatives and hedged items in fair value hedging relationships	[4]		$ 15		$ 34

[1] Excludes the interest income/expense of the respective hedged items.

[2] Represents interest income/expense on derivatives in qualifying fair-value hedging relationships. Net interest settlements on derivatives that are not in qualifying fair-value hedging relationships are reported in other income.

[3] Prior period amounts do not include amortization on the hedged items.

[4] Prior period amounts were not conformed to new hedge accounting guidance adopted January 1, 2019.