Investment Securities - Amortized Costs and Estimated Fair Values (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Available-for-sale securities
Amortized cost	$ 378,756,000,000		$ 381,729,000,000
Gross unrealized gains	4,904,000,000		6,705,000,000
Gross unrealized losses	3,718,000,000		256,000,000
Fair value	379,942,000,000		388,178,000,000
Held-to-maturity securities
Amortized cost	217,452,000,000		201,821,000,000
Gross unrealized gains	2,569,000,000		3,712,000,000
Gross unrealized losses	2,311,000,000		61,000,000
Fair value	217,710,000,000		205,472,000,000
Total investment securities, net of allowance for credit losses
Amortized cost	596,208,000,000		583,550,000,000
Gross unrealized gains	7,473,000,000		10,417,000,000
Gross unrealized losses	6,029,000,000		317,000,000
Fair value	597,652,000,000		593,650,000,000
Allowance for credit losses on AFS securities	0		0
HTM securities purchased	31,300,000,000	$ 205,000,000
HTM securities, allowance for credit losses	94,000,000	19,000,000	78,000,000
Accrued interest receivables on HTM securities	1,800,000,000		2,100,000,000
Accrued interest receivables reversed through interest income	0	$ 0
Total mortgage-backed securities
Available-for-sale securities
Amortized cost	125,755,000,000		123,795,000,000
Gross unrealized gains	1,742,000,000		2,687,000,000
Gross unrealized losses	1,408,000,000		92,000,000
Fair value	126,089,000,000		126,390,000,000
Held-to-maturity securities
Amortized cost	105,092,000,000		114,836,000,000
Gross unrealized gains	2,042,000,000		3,053,000,000
Gross unrealized losses	681,000,000		59,000,000
Fair value	106,453,000,000		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	108,200,000,000
Securities exceeding 10% of total stockholders' equity, Fair value	108,700,000,000
Total mortgage-backed securities | Freddie Mac
Total investment securities, net of allowance for credit losses
Securities exceeding 10% of total stockholders' equity, Amortized cost	31,900,000,000
Securities exceeding 10% of total stockholders' equity, Fair value	32,300,000,000
U.S. GSEs and government agencies
Available-for-sale securities
Amortized cost	112,356,000,000		110,979,000,000
Gross unrealized gains	1,479,000,000		2,372,000,000
Gross unrealized losses	1,376,000,000		50,000,000
Fair value	112,459,000,000		113,301,000,000
Held-to-maturity securities
Amortized cost	97,626,000,000		107,889,000,000
Gross unrealized gains	2,020,000,000		2,968,000,000
Gross unrealized losses	630,000,000		29,000,000
Fair value	99,016,000,000		110,828,000,000
Residential: U.S.
Available-for-sale securities
Amortized cost	5,788,000,000		6,246,000,000
Gross unrealized gains	180,000,000		224,000,000
Gross unrealized losses	5,000,000		3,000,000
Fair value	5,963,000,000		6,467,000,000
Held-to-maturity securities
Amortized cost	4,640,000,000		4,345,000,000
Gross unrealized gains	3,000,000		8,000,000
Gross unrealized losses	43,000,000		30,000,000
Fair value	4,600,000,000		4,323,000,000
Residential: Non-U.S.
Available-for-sale securities
Amortized cost	4,607,000,000		3,751,000,000
Gross unrealized gains	31,000,000		20,000,000
Gross unrealized losses	0		5,000,000
Fair value	4,638,000,000		3,766,000,000
Commercial
Available-for-sale securities
Amortized cost	3,004,000,000		2,819,000,000
Gross unrealized gains	52,000,000		71,000,000
Gross unrealized losses	27,000,000		34,000,000
Fair value	3,029,000,000		2,856,000,000
Held-to-maturity securities
Amortized cost	2,826,000,000		2,602,000,000
Gross unrealized gains	19,000,000		77,000,000
Gross unrealized losses	8,000,000		0
Fair value	2,837,000,000		2,679,000,000
U.S. Treasury and government agencies
Available-for-sale securities
Amortized cost	195,423,000,000		199,910,000,000
Gross unrealized gains	1,586,000,000		2,141,000,000
Gross unrealized losses	2,225,000,000		100,000,000
Fair value	194,784,000,000		201,951,000,000
Held-to-maturity securities
Amortized cost	76,107,000,000		53,184,000,000
Gross unrealized gains	13,000,000		50,000,000
Gross unrealized losses	1,575,000,000		0
Fair value	74,545,000,000		53,234,000,000
Obligations of U.S. states and municipalities
Available-for-sale securities
Amortized cost	18,651,000,000		18,993,000,000
Gross unrealized gains	1,276,000,000		1,404,000,000
Gross unrealized losses	5,000,000		1,000,000
Fair value	19,922,000,000		20,396,000,000
Held-to-maturity securities
Amortized cost	12,658,000,000		12,751,000,000
Gross unrealized gains	389,000,000		519,000,000
Gross unrealized losses	52,000,000		0
Fair value	12,995,000,000		13,270,000,000
Non-U.S. government debt securities
Available-for-sale securities
Amortized cost	20,745,000,000		22,587,000,000
Gross unrealized gains	192,000,000		354,000,000
Gross unrealized losses	64,000,000		13,000,000
Fair value	20,873,000,000		22,928,000,000
Corporate debt securities
Available-for-sale securities
Amortized cost	210,000,000		215,000,000
Gross unrealized gains	4,000,000		4,000,000
Gross unrealized losses	4,000,000		3,000,000
Fair value	210,000,000		216,000,000
Asset-backed securities: Collateralized loan obligations
Available-for-sale securities
Amortized cost	11,309,000,000		10,055,000,000
Gross unrealized gains	25,000,000		24,000,000
Gross unrealized losses	5,000,000		31,000,000
Fair value	11,329,000,000		10,048,000,000
Held-to-maturity securities
Amortized cost	23,595,000,000		21,050,000,000
Gross unrealized gains	125,000,000		90,000,000
Gross unrealized losses	3,000,000		2,000,000
Fair value	23,717,000,000		21,138,000,000
Asset-backed securities: Other
Available-for-sale securities
Amortized cost	6,663,000,000		6,174,000,000
Gross unrealized gains	79,000,000		91,000,000
Gross unrealized losses	7,000,000		16,000,000
Fair value	6,735,000,000		6,249,000,000
U.S. GSE obligations
Available-for-sale securities
Fair value	61,000,000,000.0		65,800,000,000
Held-to-maturity securities
Amortized cost	$ 78,800,000,000		$ 86,300,000,000