Investment Securities - Amortized Costs, Fair Value (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Available-for-sale securities
Amortized cost	$ 381,729,000,000	$ 345,306,000,000
Gross unrealized gains	6,705,000,000	5,771,000,000
Gross unrealized losses	256,000,000	378,000,000
Fair value	388,178,000,000	350,699,000,000
Held-to-maturity securities
Amortized cost	201,821,000,000	47,540,000,000
Gross unrealized gains	3,712,000,000	1,464,000,000
Gross unrealized losses	61,000,000	63,000,000
Fair value	205,472,000,000	48,941,000,000
Total investment securities, net of allowance for credit losses
Amortized cost	583,550,000,000	392,846,000,000
Gross unrealized gains	10,417,000,000	7,235,000,000
Gross unrealized losses	317,000,000	441,000,000
Fair value	593,650,000,000	399,640,000,000
Allowance for credit losses on AFS securities	0
HTM securities purchased	12,400,000,000	13,400,000,000	$ 9,400,000,000
Held-to-maturity securities, allowance for credit losses	78,000,000
Accrued interest receivables on HTM securities	2,100,000,000	1,900,000,000
Accrued interest receivables reversed through interest income	0	0
Total mortgage-backed securities
Available-for-sale securities
Amortized cost	123,795,000,000	125,648,000,000
Gross unrealized gains	2,687,000,000	2,756,000,000
Gross unrealized losses	92,000,000	109,000,000
Fair value	126,390,000,000	128,295,000,000
Held-to-maturity securities
Amortized cost	114,836,000,000	36,523,000,000
Gross unrealized gains	3,053,000,000	1,165,000,000
Gross unrealized losses	59,000,000	62,000,000
Fair value	117,830,000,000	37,626,000,000
Total mortgage-backed securities | Fannie Mae
Total investment securities, net of allowance for credit losses
Securities exceeding 10% of total stockholders' equity, Amortized cost	95,700,000,000
Securities exceeding 10% of total stockholders' equity, Fair value	98,800,000,000
Total mortgage-backed securities | Freddie Mac
Total investment securities, net of allowance for credit losses
Securities exceeding 10% of total stockholders' equity, Amortized cost	54,700,000,000
Securities exceeding 10% of total stockholders' equity, Fair value	55,800,000,000
U.S. GSEs and government agencies
Available-for-sale securities
Amortized cost	110,979,000,000	107,811,000,000
Gross unrealized gains	2,372,000,000	2,395,000,000
Gross unrealized losses	50,000,000	89,000,000
Fair value	113,301,000,000	110,117,000,000
Held-to-maturity securities
Amortized cost	107,889,000,000	36,523,000,000
Gross unrealized gains	2,968,000,000	1,165,000,000
Gross unrealized losses	29,000,000	62,000,000
Fair value	110,828,000,000	37,626,000,000
Residential: U.S.
Available-for-sale securities
Amortized cost	6,246,000,000	10,223,000,000
Gross unrealized gains	224,000,000	233,000,000
Gross unrealized losses	3,000,000	6,000,000
Fair value	6,467,000,000	10,450,000,000
Held-to-maturity securities
Amortized cost	4,345,000,000	0
Gross unrealized gains	8,000,000	0
Gross unrealized losses	30,000,000	0
Fair value	4,323,000,000	0
Residential: Non-U.S.
Available-for-sale securities
Amortized cost	3,751,000,000	2,477,000,000
Gross unrealized gains	20,000,000	64,000,000
Gross unrealized losses	5,000,000	1,000,000
Fair value	3,766,000,000	2,540,000,000
Commercial
Available-for-sale securities
Amortized cost	2,819,000,000	5,137,000,000
Gross unrealized gains	71,000,000	64,000,000
Gross unrealized losses	34,000,000	13,000,000
Fair value	2,856,000,000	5,188,000,000
Held-to-maturity securities
Amortized cost	2,602,000,000	0
Gross unrealized gains	77,000,000	0
Gross unrealized losses	0	0
Fair value	2,679,000,000	0
U.S. Treasury and government agencies
Available-for-sale securities
Amortized cost	199,910,000,000	139,162,000,000
Gross unrealized gains	2,141,000,000	449,000,000
Gross unrealized losses	100,000,000	175,000,000
Fair value	201,951,000,000	139,436,000,000
Held-to-maturity securities
Amortized cost	53,184,000,000	51,000,000
Gross unrealized gains	50,000,000	0
Gross unrealized losses	0	1,000,000
Fair value	53,234,000,000	50,000,000
Obligations of U.S. states and municipalities
Available-for-sale securities
Amortized cost	18,993,000,000	27,693,000,000
Gross unrealized gains	1,404,000,000	2,118,000,000
Gross unrealized losses	1,000,000	1,000,000
Fair value	20,396,000,000	29,810,000,000
Held-to-maturity securities
Amortized cost	12,751,000,000	4,797,000,000
Gross unrealized gains	519,000,000	299,000,000
Gross unrealized losses	0	0
Fair value	13,270,000,000	5,096,000,000
Certificates of deposit
Available-for-sale securities
Amortized cost	0	77,000,000
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair value	0	77,000,000
Non-U.S. government debt securities
Available-for-sale securities
Amortized cost	22,587,000,000	21,427,000,000
Gross unrealized gains	354,000,000	377,000,000
Gross unrealized losses	13,000,000	17,000,000
Fair value	22,928,000,000	21,787,000,000
Corporate debt securities
Available-for-sale securities
Amortized cost	215,000,000	823,000,000
Gross unrealized gains	4,000,000	22,000,000
Gross unrealized losses	3,000,000	0
Fair value	216,000,000	845,000,000
Asset-backed securities: Collateralized loan obligations
Available-for-sale securities
Amortized cost	10,055,000,000	25,038,000,000
Gross unrealized gains	24,000,000	9,000,000
Gross unrealized losses	31,000,000	56,000,000
Fair value	10,048,000,000	24,991,000,000
Held-to-maturity securities
Amortized cost	21,050,000,000	6,169,000,000
Gross unrealized gains	90,000,000	0
Gross unrealized losses	2,000,000	0
Fair value	21,138,000,000	6,169,000,000
Asset-backed securities: Other
Available-for-sale securities
Amortized cost	6,174,000,000	5,438,000,000
Gross unrealized gains	91,000,000	40,000,000
Gross unrealized losses	16,000,000	20,000,000
Fair value	6,249,000,000	5,458,000,000
US GSE obligations
Available-for-sale securities
Fair value	65,800,000,000	78,500,000,000
Held-to-maturity securities
Amortized cost	$ 86,300,000,000	$ 31,600,000,000