Fair Value (Fair Value on a Recurring Basis) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	$ 1,728	$ 888
Available-for-sale securities	16,166	16,651
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	162	63
Derivative assets, net	233	102
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	447	201
Derivative liabilities, net	(9)	(1)
Fair Value, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
Available-for-sale securities	0	0
Derivative assets, net	0	0
Other assets	28	34
Derivative liabilities, net	0	0
Fair Value, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	1,728	888
Available-for-sale securities	16,166	16,651
Derivative assets, net	71	39
Other assets	0	0
Derivative liabilities, net	(456)	(202)
Fair Value, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
Available-for-sale securities	0	0
Derivative assets, net	0	0
Other assets	0	0
Derivative liabilities, net	0	0
Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	162	63
Derivative assets, net	233
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	447	201
Derivative liabilities, net	(9)
Fair Value, Recurring [Member] | Fair Value, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
Available-for-sale securities	0	0
Derivative assets, net	0
Other assets	28	34
Total recurring assets	28	34
Derivative liabilities, net	0
Total recurring liabilities	0	0
Fair Value, Recurring [Member] | Fair Value, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	1,728	888
Available-for-sale securities	16,166	16,651
Derivative assets, net	71
Other assets	0	0
Total recurring assets	17,965	17,578
Derivative liabilities, net	(456)
Total recurring liabilities	(456)	(202)
Fair Value, Recurring [Member] | Fair Value, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
Available-for-sale securities	0	0
Derivative assets, net	0
Other assets	0	0
Total recurring assets	0	0
Derivative liabilities, net	0
Total recurring liabilities	0	0
Interest Rate Swap [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets, net	162	63
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	447	201
Interest Rate Swap [Member] | Fair Value, Recurring [Member] | Fair Value, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets, net	0	0
Derivative liabilities, net	0	0
Interest Rate Swap [Member] | Fair Value, Recurring [Member] | Fair Value, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets, net	67	39
Derivative liabilities, net	(449)	(202)
Interest Rate Swap [Member] | Fair Value, Recurring [Member] | Fair Value, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets, net	0	0
Derivative liabilities, net	0	0
Forward Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets, net	0
Derivative liabilities, net	0
Forward Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets, net	4
Derivative liabilities, net	(7)
Forward Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets, net	0
Derivative liabilities, net	0
Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	1,728	888
Available-for-sale securities	16,166	16,651
Derivative assets, net	233	102
Other assets	28	34
Derivative liabilities, net	(9)	(1)
Fair Value [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	1,728	888
Available-for-sale securities	16,166	16,651
Other assets	28	34
Total recurring assets	18,155	17,675
Total recurring liabilities	(9)	(1)
Fair Value [Member] | Interest Rate Swap [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets, net	229	102
Derivative liabilities, net	(2)	(1)
Fair Value [Member] | Forward Contracts [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets, net	4
Derivative liabilities, net	(7)
US Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	873	0
US Treasury Securities [Member] | Fair Value, Recurring [Member] | Fair Value, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0
US Treasury Securities [Member] | Fair Value, Recurring [Member] | Fair Value, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	873
US Treasury Securities [Member] | Fair Value, Recurring [Member] | Fair Value, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0
US Treasury Securities [Member] | Fair Value [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	873
U.S. obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	140	150
Available-for-sale securities	1,975	2,127
U.S. obligations [Member] | Fair Value, Recurring [Member] | Fair Value, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
Available-for-sale securities	0	0
U.S. obligations [Member] | Fair Value, Recurring [Member] | Fair Value, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	140	150
Available-for-sale securities	1,975	2,127
U.S. obligations [Member] | Fair Value, Recurring [Member] | Fair Value, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
Available-for-sale securities	0	0
U.S. obligations [Member] | Fair Value [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	140	150
Available-for-sale securities	1,975	2,127
GSE obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	64	60
Available-for-sale securities	1,107	1,060
GSE obligations [Member] | Fair Value, Recurring [Member] | Fair Value, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
Available-for-sale securities	0	0
GSE obligations [Member] | Fair Value, Recurring [Member] | Fair Value, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	64	60
Available-for-sale securities	1,107	1,060
GSE obligations [Member] | Fair Value, Recurring [Member] | Fair Value, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
Available-for-sale securities	0	0
GSE obligations [Member] | Fair Value [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	64	60
Available-for-sale securities	1,107	1,060
State or local housing agency obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	760	756
State or local housing agency obligations [Member] | Fair Value, Recurring [Member] | Fair Value, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
State or local housing agency obligations [Member] | Fair Value, Recurring [Member] | Fair Value, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	760	756
State or local housing agency obligations [Member] | Fair Value, Recurring [Member] | Fair Value, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
State or local housing agency obligations [Member] | Fair Value [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	760	756
Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	262	259
Available-for-sale securities	292	285
Other [Member] | Fair Value, Recurring [Member] | Fair Value, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
Available-for-sale securities	0	0
Other [Member] | Fair Value, Recurring [Member] | Fair Value, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	262	259
Available-for-sale securities	292	285
Other [Member] | Fair Value, Recurring [Member] | Fair Value, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
Available-for-sale securities	0	0
Other [Member] | Fair Value [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	262	259
Available-for-sale securities	292	285
U.S. obligations MBS [Member] | Single Family [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	3,892	4,059
U.S. obligations MBS [Member] | Single Family [Member] | Fair Value, Recurring [Member] | Fair Value, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
U.S. obligations MBS [Member] | Single Family [Member] | Fair Value, Recurring [Member] | Fair Value, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	3,892	4,059
U.S. obligations MBS [Member] | Single Family [Member] | Fair Value, Recurring [Member] | Fair Value, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
U.S. obligations MBS [Member] | Single Family [Member] | Fair Value [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	3,892	4,059
Mortgage-backed securities, GSE [Member] | Single Family [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	609	649
Mortgage-backed securities, GSE [Member] | Single Family [Member] | Fair Value, Recurring [Member] | Fair Value, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Mortgage-backed securities, GSE [Member] | Single Family [Member] | Fair Value, Recurring [Member] | Fair Value, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	609	649
Mortgage-backed securities, GSE [Member] | Single Family [Member] | Fair Value, Recurring [Member] | Fair Value, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Mortgage-backed securities, GSE [Member] | Single Family [Member] | Fair Value [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	609	649
Mortgage-backed securities, GSE [Member] | Multifamily [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	389	419
Available-for-sale securities	7,531	7,715
Mortgage-backed securities, GSE [Member] | Multifamily [Member] | Fair Value, Recurring [Member] | Fair Value, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
Available-for-sale securities	0	0
Mortgage-backed securities, GSE [Member] | Multifamily [Member] | Fair Value, Recurring [Member] | Fair Value, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	389	419
Available-for-sale securities	7,531	7,715
Mortgage-backed securities, GSE [Member] | Multifamily [Member] | Fair Value, Recurring [Member] | Fair Value, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
Available-for-sale securities	0	0
Mortgage-backed securities, GSE [Member] | Multifamily [Member] | Fair Value [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	389	419
Available-for-sale securities	$ 7,531	$ 7,715