Investments (AFS Major Security Types) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 16,282	$ 16,603
Gross Unrealized Gains	14	75
Gross Unrealized Losses	(130)	(27)
Fair Value	16,166	16,651
Debt Securities, Available-for-Sale, Accrued Interest, after Allowance for Credit Loss	33	42
U.S. obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,000	2,122
Gross Unrealized Gains	2	6
Gross Unrealized Losses	(27)	(1)
Fair Value	1,975	2,127
GSE obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,118	1,034
Gross Unrealized Gains	5	26
Gross Unrealized Losses	(16)	0
Fair Value	1,107	1,060
State or local housing agency obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	761	761
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	(5)
Fair Value	760	756
Other Debt Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	292	276
Gross Unrealized Gains	1	9
Gross Unrealized Losses	(1)	0
Fair Value	292	285
Non-mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,171	4,193
Gross Unrealized Gains	8	41
Gross Unrealized Losses	(45)	(6)
Fair Value	4,134	4,228
Mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	12,111	12,410
Gross Unrealized Gains	6	34
Gross Unrealized Losses	(85)	(21)
Fair Value	12,032	12,423
Single Family [Member] \| U.S. obligations MBS [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,917	4,044
Gross Unrealized Gains	1	17
Gross Unrealized Losses	(26)	(2)
Fair Value	3,892	4,059
Single Family [Member] \| Mortgage-backed securities, GSE [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	609	646
Gross Unrealized Gains	3	4
Gross Unrealized Losses	(3)	(1)
Fair Value	609	649
Multifamily [Member] \| Mortgage-backed securities, GSE [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,585	7,720
Gross Unrealized Gains	2	13
Gross Unrealized Losses	(56)	(18)
Fair Value	$ 7,531	$ 7,715

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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