Investment Securities - Amortized Costs and Estimated Fair Values (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Available-for-sale securities
Amortized cost	$ 230,110,000,000		$ 230,110,000,000		$ 381,729,000,000
Gross unrealized gains	3,458,000,000		3,458,000,000		6,705,000,000
Gross unrealized losses	1,407,000,000		1,407,000,000		256,000,000
Fair value	232,161,000,000		232,161,000,000		388,178,000,000
Held-to-maturity securities
Amortized cost	341,476,000,000		341,476,000,000		201,821,000,000
Gross unrealized gains	2,829,000,000		2,829,000,000		3,712,000,000
Gross unrealized losses	1,452,000,000		1,452,000,000		61,000,000
Fair value	342,853,000,000		342,853,000,000		205,472,000,000
Total investment securities, net of allowance for credit losses
Amortized cost	571,586,000,000		571,586,000,000		583,550,000,000
Gross unrealized gains	6,287,000,000		6,287,000,000		10,417,000,000
Gross unrealized losses	2,859,000,000		2,859,000,000		317,000,000
Fair value	575,014,000,000		575,014,000,000		593,650,000,000
HTM securities purchased	31,800,000,000	$ 4,800,000,000	63,100,000,000	$ 5,000,000,000.0
Investment securities, allowance for credit losses	87,000,000	23,000,000	87,000,000	23,000,000	78,000,000
Accrued interest receivables on HTM securities	1,800,000,000		1,800,000,000		2,100,000,000
Accrued interest receivables reversed through interest income	0	$ 0	0	$ 0
Total mortgage-backed securities
Available-for-sale securities
Amortized cost	81,386,000,000		81,386,000,000		123,795,000,000
Gross unrealized gains	1,321,000,000		1,321,000,000		2,687,000,000
Gross unrealized losses	584,000,000		584,000,000		92,000,000
Fair value	82,123,000,000		82,123,000,000		126,390,000,000
Held-to-maturity securities
Amortized cost	115,926,000,000		115,926,000,000		114,836,000,000
Gross unrealized gains	1,888,000,000		1,888,000,000		3,053,000,000
Gross unrealized losses	436,000,000		436,000,000		59,000,000
Fair value	117,378,000,000		117,378,000,000		117,830,000,000
Total mortgage-backed securities | Fannie Mae
Total investment securities, net of allowance for credit losses
Securities exceeding 10% of total stockholders' equity, Amortized cost	73,800,000,000		73,800,000,000
Securities exceeding 10% of total stockholders' equity, Fair value	75,500,000,000		75,500,000,000
Total mortgage-backed securities | Freddie Mac
Total investment securities, net of allowance for credit losses
Securities exceeding 10% of total stockholders' equity, Amortized cost	44,300,000,000		44,300,000,000
Securities exceeding 10% of total stockholders' equity, Fair value	44,500,000,000		44,500,000,000
U.S. GSEs and government agencies
Available-for-sale securities
Amortized cost	72,088,000,000		72,088,000,000		110,979,000,000
Gross unrealized gains	1,168,000,000		1,168,000,000		2,372,000,000
Gross unrealized losses	574,000,000		574,000,000		50,000,000
Fair value	72,682,000,000		72,682,000,000		113,301,000,000
Held-to-maturity securities
Amortized cost	105,539,000,000		105,539,000,000		107,889,000,000
Gross unrealized gains	1,855,000,000		1,855,000,000		2,968,000,000
Gross unrealized losses	396,000,000		396,000,000		29,000,000
Fair value	106,998,000,000		106,998,000,000		110,828,000,000
Residential: U.S.
Available-for-sale securities
Amortized cost	2,526,000,000		2,526,000,000		6,246,000,000
Gross unrealized gains	73,000,000		73,000,000		224,000,000
Gross unrealized losses	0		0		3,000,000
Fair value	2,599,000,000		2,599,000,000		6,467,000,000
Held-to-maturity securities
Amortized cost	6,805,000,000		6,805,000,000		4,345,000,000
Gross unrealized gains	5,000,000		5,000,000		8,000,000
Gross unrealized losses	35,000,000		35,000,000		30,000,000
Fair value	6,775,000,000		6,775,000,000		4,323,000,000
Residential: Non-U.S.
Available-for-sale securities
Amortized cost	4,335,000,000		4,335,000,000		3,751,000,000
Gross unrealized gains	36,000,000		36,000,000		20,000,000
Gross unrealized losses	0		0		5,000,000
Fair value	4,371,000,000		4,371,000,000		3,766,000,000
Commercial
Available-for-sale securities
Amortized cost	2,437,000,000		2,437,000,000		2,819,000,000
Gross unrealized gains	44,000,000		44,000,000		71,000,000
Gross unrealized losses	10,000,000		10,000,000		34,000,000
Fair value	2,471,000,000		2,471,000,000		2,856,000,000
Held-to-maturity securities
Amortized cost	3,582,000,000		3,582,000,000		2,602,000,000
Gross unrealized gains	28,000,000		28,000,000		77,000,000
Gross unrealized losses	5,000,000		5,000,000		0
Fair value	3,605,000,000		3,605,000,000		2,679,000,000
U.S. Treasury and government agencies
Available-for-sale securities
Amortized cost	99,905,000,000		99,905,000,000		199,910,000,000
Gross unrealized gains	713,000,000		713,000,000		2,141,000,000
Gross unrealized losses	771,000,000		771,000,000		100,000,000
Fair value	99,847,000,000		99,847,000,000		201,951,000,000
Held-to-maturity securities
Amortized cost	182,529,000,000		182,529,000,000		53,184,000,000
Gross unrealized gains	338,000,000		338,000,000		50,000,000
Gross unrealized losses	1,012,000,000		1,012,000,000		0
Fair value	181,855,000,000		181,855,000,000		53,234,000,000
Obligations of U.S. states and municipalities
Available-for-sale securities
Amortized cost	17,249,000,000		17,249,000,000		18,993,000,000
Gross unrealized gains	1,208,000,000		1,208,000,000		1,404,000,000
Gross unrealized losses	2,000,000		2,000,000		1,000,000
Fair value	18,455,000,000		18,455,000,000		20,396,000,000
Held-to-maturity securities
Amortized cost	13,515,000,000		13,515,000,000		12,751,000,000
Gross unrealized gains	493,000,000		493,000,000		519,000,000
Gross unrealized losses	0		0		0
Fair value	14,008,000,000		14,008,000,000		13,270,000,000
Non-U.S. government debt securities
Available-for-sale securities
Amortized cost	17,649,000,000		17,649,000,000		22,587,000,000
Gross unrealized gains	128,000,000		128,000,000		354,000,000
Gross unrealized losses	37,000,000		37,000,000		13,000,000
Fair value	17,740,000,000		17,740,000,000		22,928,000,000
Corporate debt securities
Available-for-sale securities
Amortized cost	214,000,000		214,000,000		215,000,000
Gross unrealized gains	9,000,000		9,000,000		4,000,000
Gross unrealized losses	7,000,000		7,000,000		3,000,000
Fair value	216,000,000		216,000,000		216,000,000
Asset-backed securities: Collateralized loan obligations
Available-for-sale securities
Amortized cost	8,808,000,000		8,808,000,000		10,055,000,000
Gross unrealized gains	12,000,000		12,000,000		24,000,000
Gross unrealized losses	4,000,000		4,000,000		31,000,000
Fair value	8,816,000,000		8,816,000,000		10,048,000,000
Held-to-maturity securities
Amortized cost	27,324,000,000		27,324,000,000		21,050,000,000
Gross unrealized gains	110,000,000		110,000,000		90,000,000
Gross unrealized losses	4,000,000		4,000,000		2,000,000
Fair value	27,430,000,000		27,430,000,000		21,138,000,000
Asset-backed securities: Other
Available-for-sale securities
Amortized cost	4,899,000,000		4,899,000,000		6,174,000,000
Gross unrealized gains	67,000,000		67,000,000		91,000,000
Gross unrealized losses	2,000,000		2,000,000		16,000,000
Fair value	4,964,000,000		4,964,000,000		6,249,000,000
Held-to-maturity securities
Amortized cost	2,182,000,000		2,182,000,000		0
Gross unrealized gains	0		0		0
Gross unrealized losses	0		0		0
Fair value	2,182,000,000		2,182,000,000		0
U.S. GSE obligations
Available-for-sale securities
Fair value	46,400,000,000		46,400,000,000		65,800,000,000
Held-to-maturity securities
Amortized cost	$ 72,200,000,000		$ 72,200,000,000		$ 86,300,000,000