Mortgage Loans and Loan Loss Reserves - Amortized Cost Basis of Single-Family Held-for-Investment Loans by Current LTV Ratios and Vintage (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable 1	$ 2,196,429	$ 1,989,146
Single-Family serious delinquency rate	3.04%	0.63%
Single-family 20 and 30-year or more, amortizing fixed-rate
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 545,387
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	293,858
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	127,346
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	148,822
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	182,351
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	541,306
Financing Receivable 1	1,839,070	$ 1,677,268
Single-family 15-year amortizing fixed-rate
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	100,512
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	34,810
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	13,995
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	22,986
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	32,961
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	92,297
Financing Receivable 1	297,561	243,723
Single-family Adjustable-rate
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	2,802
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	2,226
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	1,624
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	4,230
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	2,797
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	15,298
Financing Receivable 1	28,977	36,909
Single-family Alt-A, interest-only, and option ARM
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	11,540
Financing Receivable 1	11,540	13,757
Single Family
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	648,701
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	330,894
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	142,965
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	176,038
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	218,109
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	660,441
Financing Receivable 1	2,177,148	$ 1,971,657
Single-Family serious delinquency rate		0.63%
Less Than Or Equal To 80 Estimated Current LTV Ratio | Single-family 20 and 30-year or more, amortizing fixed-rate
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	378,844
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	198,519
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	101,325
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	141,534
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	180,428
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	533,138
Financing Receivable 1	1,533,788	$ 1,405,562
Less Than Or Equal To 80 Estimated Current LTV Ratio | Single-family 15-year amortizing fixed-rate
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	90,860
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	33,429
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	13,902
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	22,955
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	32,940
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	92,255
Financing Receivable 1	286,341	236,837
Less Than Or Equal To 80 Estimated Current LTV Ratio | Single-family Adjustable-rate
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	2,453
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,960
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	1,525
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	4,149
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	2,786
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	15,281
Financing Receivable 1	28,154	35,478
Less Than Or Equal To 80 Estimated Current LTV Ratio | Single-family Alt-A, interest-only, and option ARM
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	10,945
Financing Receivable 1	10,945	12,668
Less Than Or Equal To 80 Estimated Current LTV Ratio | Single Family
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	472,157
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	233,908
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	116,752
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	168,638
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	216,154
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	651,619
Financing Receivable 1	1,859,228	1,690,545
Greater Than 80 Through 100 Estimated Current LTV Ratio | Single-family 20 and 30-year or more, amortizing fixed-rate
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	166,082
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	95,300
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	25,976
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	7,190
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	1,829
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	6,682
Financing Receivable 1	303,059	267,752
Greater Than 80 Through 100 Estimated Current LTV Ratio | Single-family 15-year amortizing fixed-rate
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	9,607
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,380
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	89
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	25
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	15
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	31
Financing Receivable 1	11,147	6,797
Greater Than 80 Through 100 Estimated Current LTV Ratio | Single-family Adjustable-rate
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	349
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	266
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	99
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	81
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	11
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	15
Financing Receivable 1	821	1,425
Greater Than 80 Through 100 Estimated Current LTV Ratio | Single-family Alt-A, interest-only, and option ARM
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	501
Financing Receivable 1	501	901
Greater Than 80 Through 100 Estimated Current LTV Ratio | Single Family
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	176,038
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	96,946
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	26,164
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	7,296
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	1,855
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	7,229
Financing Receivable 1	315,528	276,875
Greater Than 100 Loan To Value Ratio [Member] | Single-family 20 and 30-year or more, amortizing fixed-rate
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	461
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	39
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	45
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	98
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	94
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	1,486
Financing Receivable 1	2,223	3,954
Greater Than 100 Loan To Value Ratio [Member] | Single-family 15-year amortizing fixed-rate
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	45
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	4
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	6
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	6
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	11
Financing Receivable 1	73	89
Greater Than 100 Loan To Value Ratio [Member] | Single-family Adjustable-rate
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	2
Financing Receivable 1	2	6
Greater Than 100 Loan To Value Ratio [Member] | Single-family Alt-A, interest-only, and option ARM
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	94
Financing Receivable 1	94	188
Greater Than 100 Loan To Value Ratio [Member] | Single Family
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	506
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	40
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	49
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	104
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	100
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	1,593
Financing Receivable 1	$ 2,392	$ 4,237
Single-Family serious delinquency rate	10.63%	4.51%