Investment Securities - Amortized Costs and Estimated Fair Values (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Available-for-sale securities
Amortized cost	$ 249,733,000,000		$ 249,733,000,000		$ 381,729,000,000
Gross unrealized gains	3,189,000,000		3,189,000,000		6,705,000,000
Gross unrealized losses	1,332,000,000		1,332,000,000		256,000,000
Fair value	251,590,000,000		251,590,000,000		388,178,000,000
Held-to-maturity securities
Amortized cost	343,542,000,000		343,542,000,000		201,821,000,000
Gross unrealized gains	2,571,000,000		2,571,000,000		3,712,000,000
Gross unrealized losses	1,720,000,000		1,720,000,000		61,000,000
Fair value	344,393,000,000		344,393,000,000		205,472,000,000
Total investment securities, net of allowance for credit losses
Amortized cost	593,275,000,000		593,275,000,000		583,550,000,000
Gross unrealized gains	5,760,000,000		5,760,000,000		10,417,000,000
Gross unrealized losses	3,052,000,000		3,052,000,000		317,000,000
Fair value	595,983,000,000		595,983,000,000		593,650,000,000
HTM securities purchased	15,900,000,000	$ 514,000,000	79,000,000,000	$ 5,500,000,000
Investment securities, allowance for credit losses	73,000,000	120,000,000	73,000,000	120,000,000	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	82,130,000,000		82,130,000,000		123,795,000,000
Gross unrealized gains	1,226,000,000		1,226,000,000		2,687,000,000
Gross unrealized losses	802,000,000		802,000,000		92,000,000
Fair value	82,554,000,000		82,554,000,000		126,390,000,000
Held-to-maturity securities
Amortized cost	111,845,000,000		111,845,000,000		114,836,000,000
Gross unrealized gains	1,947,000,000		1,947,000,000		3,053,000,000
Gross unrealized losses	476,000,000		476,000,000		59,000,000
Fair value	113,316,000,000		113,316,000,000		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	69,500,000,000		69,500,000,000
Securities exceeding 10% of total stockholders' equity, Fair value	71,100,000,000		71,100,000,000
Total mortgage-backed securities | Freddie Mac
Total investment securities, net of allowance for credit losses
Securities exceeding 10% of total stockholders' equity, Amortized cost	46,300,000,000		46,300,000,000
Securities exceeding 10% of total stockholders' equity, Fair value	46,300,000,000		46,300,000,000
U.S. GSEs and government agencies
Available-for-sale securities
Amortized cost	72,749,000,000		72,749,000,000		110,979,000,000
Gross unrealized gains	1,091,000,000		1,091,000,000		2,372,000,000
Gross unrealized losses	791,000,000		791,000,000		50,000,000
Fair value	73,049,000,000		73,049,000,000		113,301,000,000
Held-to-maturity securities
Amortized cost	101,086,000,000		101,086,000,000		107,889,000,000
Gross unrealized gains	1,923,000,000		1,923,000,000		2,968,000,000
Gross unrealized losses	412,000,000		412,000,000		29,000,000
Fair value	102,597,000,000		102,597,000,000		110,828,000,000
Residential: U.S.
Available-for-sale securities
Amortized cost	2,265,000,000		2,265,000,000		6,246,000,000
Gross unrealized gains	58,000,000		58,000,000		224,000,000
Gross unrealized losses	0		0		3,000,000
Fair value	2,323,000,000		2,323,000,000		6,467,000,000
Held-to-maturity securities
Amortized cost	7,021,000,000		7,021,000,000		4,345,000,000
Gross unrealized gains	4,000,000		4,000,000		8,000,000
Gross unrealized losses	43,000,000		43,000,000		30,000,000
Fair value	6,982,000,000		6,982,000,000		4,323,000,000
Residential: Non-U.S.
Available-for-sale securities
Amortized cost	4,014,000,000		4,014,000,000		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	4,050,000,000		4,050,000,000		3,766,000,000
Commercial
Available-for-sale securities
Amortized cost	3,102,000,000		3,102,000,000		2,819,000,000
Gross unrealized gains	41,000,000		41,000,000		71,000,000
Gross unrealized losses	11,000,000		11,000,000		34,000,000
Fair value	3,132,000,000		3,132,000,000		2,856,000,000
Held-to-maturity securities
Amortized cost	3,738,000,000		3,738,000,000		2,602,000,000
Gross unrealized gains	20,000,000		20,000,000		77,000,000
Gross unrealized losses	21,000,000		21,000,000		0
Fair value	3,737,000,000		3,737,000,000		2,679,000,000
U.S. Treasury and government agencies
Available-for-sale securities
Amortized cost	121,227,000,000		121,227,000,000		199,910,000,000
Gross unrealized gains	730,000,000		730,000,000		2,141,000,000
Gross unrealized losses	486,000,000		486,000,000		100,000,000
Fair value	121,471,000,000		121,471,000,000		201,951,000,000
Held-to-maturity securities
Amortized cost	182,368,000,000		182,368,000,000		53,184,000,000
Gross unrealized gains	133,000,000		133,000,000		50,000,000
Gross unrealized losses	1,225,000,000		1,225,000,000		0
Fair value	181,276,000,000		181,276,000,000		53,234,000,000
Obligations of U.S. states and municipalities
Available-for-sale securities
Amortized cost	16,471,000,000		16,471,000,000		18,993,000,000
Gross unrealized gains	1,049,000,000		1,049,000,000		1,404,000,000
Gross unrealized losses	5,000,000		5,000,000		1,000,000
Fair value	17,515,000,000		17,515,000,000		20,396,000,000
Held-to-maturity securities
Amortized cost	13,465,000,000		13,465,000,000		12,751,000,000
Gross unrealized gains	397,000,000		397,000,000		519,000,000
Gross unrealized losses	9,000,000		9,000,000		0
Fair value	13,853,000,000		13,853,000,000		13,270,000,000
Non-U.S. government debt securities
Available-for-sale securities
Amortized cost	15,738,000,000		15,738,000,000		22,587,000,000
Gross unrealized gains	111,000,000		111,000,000		354,000,000
Gross unrealized losses	24,000,000		24,000,000		13,000,000
Fair value	15,825,000,000		15,825,000,000		22,928,000,000
Corporate debt securities
Available-for-sale securities
Amortized cost	263,000,000		263,000,000		215,000,000
Gross unrealized gains	6,000,000		6,000,000		4,000,000
Gross unrealized losses	9,000,000		9,000,000		3,000,000
Fair value	260,000,000		260,000,000		216,000,000
Asset-backed securities: Collateralized loan obligations
Available-for-sale securities
Amortized cost	9,133,000,000		9,133,000,000		10,055,000,000
Gross unrealized gains	8,000,000		8,000,000		24,000,000
Gross unrealized losses	4,000,000		4,000,000		31,000,000
Fair value	9,137,000,000		9,137,000,000		10,048,000,000
Held-to-maturity securities
Amortized cost	33,664,000,000		33,664,000,000		21,050,000,000
Gross unrealized gains	90,000,000		90,000,000		90,000,000
Gross unrealized losses	9,000,000		9,000,000		2,000,000
Fair value	33,745,000,000		33,745,000,000		21,138,000,000
Asset-backed securities: Other
Available-for-sale securities
Amortized cost	4,771,000,000		4,771,000,000		6,174,000,000
Gross unrealized gains	59,000,000		59,000,000		91,000,000
Gross unrealized losses	2,000,000		2,000,000		16,000,000
Fair value	4,828,000,000		4,828,000,000		6,249,000,000
Held-to-maturity securities
Amortized cost	2,200,000,000		2,200,000,000		0
Gross unrealized gains	4,000,000		4,000,000		0
Gross unrealized losses	1,000,000		1,000,000		0
Fair value	2,203,000,000		2,203,000,000		0
U.S. GSE obligations
Available-for-sale securities
Fair value	44,100,000,000		44,100,000,000		65,800,000,000
Held-to-maturity securities
Amortized cost	$ 71,900,000,000		$ 71,900,000,000		$ 86,300,000,000