Fair Value Level 3 Valuation Inputs - Recurring (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at fair value	$ 6,490	$ 7,825
Net derivatives	1,046	231
Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	136,542	48,123
Available-for-sale securities	1,697	2,404
Mortgage loans held for investment, at fair value	6,490	7,825
Long-term debt	(28,314)	(27,567)
Total nonrecurring assets at fair value	147,074	58,623
Fair Value, Inputs, Level 1 [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	130,456	39,501
Available-for-sale securities	0	0
Mortgage loans held for investment, at fair value	0	0
Mortgage loans held for sale, at lower of cost or fair value	0	0
Total nonrecurring assets at fair value	227,635	89,558
Fair Value, Inputs, Level 1 [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	130,456	39,501
Available-for-sale securities	0	0
Mortgage loans held for investment, at fair value	0	0
Long-term debt	0	0
Total nonrecurring assets at fair value	131,576	39,501
Fair Value, Inputs, Level 1 [Member] | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Total nonrecurring assets at fair value	0	0
Fair Value, Inputs, Level 2 [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	5,991	8,576
Available-for-sale securities	1,049	1,612
Mortgage loans held for investment, at fair value	3,512,672	3,270,535
Mortgage loans held for sale, at lower of cost or fair value	116	229
Total nonrecurring assets at fair value	3,578,668	3,313,691
Fair Value, Inputs, Level 2 [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	5,991	8,576
Available-for-sale securities	1,049	1,612
Mortgage loans held for investment, at fair value	5,629	7,137
Long-term debt	(27,815)	(27,094)
Total nonrecurring assets at fair value	14,417	18,685
Fair Value, Inputs, Level 2 [Member] | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for sale, at lower of cost or fair value	25	274
Other Liabilities, Fair Value Disclosure	0	0
Significant Unobservable Inputs (Level 3) [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	95	46
Available-for-sale securities	648	792
Mortgage loans held for investment, at fair value	255,556	127,650
Mortgage loans held for sale, at lower of cost or fair value	5,502	7,054
Total nonrecurring assets at fair value	262,442	136,048
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	95	46
Available-for-sale securities	648	792
Mortgage loans held for investment, at fair value	861	688
Net derivatives	333	162
Long-term debt	(499)	(473)
Total nonrecurring assets at fair value	1,986	1,725
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Other valuation technique [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Net derivatives	130	15
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Dealer Mark [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Net derivatives	203	147
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for sale, at lower of cost or fair value	1,087	1,076
Total nonrecurring assets at fair value	2,742	2,598
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Nonrecurring [Member] | Consensus without inputs [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for sale, at lower of cost or fair value	754	471
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Nonrecurring [Member] | Single vendor without inputs [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for sale, at lower of cost or fair value	333	605
Agency [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	195
Agency [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Consensus without inputs [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	97
Agency [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Other valuation technique [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	95	46
Available-for-sale securities	98	107
Alt-A and subprime private-label securities [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	158	629
Available-for-sale securities	6	57
Alt-A and subprime private-label securities [Member] | Fair Value, Inputs, Level 1 [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	0	0
Available-for-sale securities	0	0
Alt-A and subprime private-label securities [Member] | Fair Value, Inputs, Level 2 [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	158	629
Available-for-sale securities	4	57
Alt-A and subprime private-label securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	0	0
Available-for-sale securities	2	0
Alt-A and subprime private-label securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Other valuation technique [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities		171
Mortgage revenue bonds [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	216	315
Mortgage revenue bonds [Member] | Fair Value, Inputs, Level 1 [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	0	0
Mortgage revenue bonds [Member] | Fair Value, Inputs, Level 2 [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	0	0
Mortgage revenue bonds [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	216	315
Mortgage revenue bonds [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Single vendor with inputs [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	144	222
Mortgage revenue bonds [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Other valuation technique [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities		64
Available-for-sale securities	72	93
Other [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	242	314
Other [Member] | Fair Value, Inputs, Level 1 [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	0	0
Other [Member] | Fair Value, Inputs, Level 2 [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	7	8
Other [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	235	306
Other [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Discounted cash flow with inputs [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	$ 206	$ 267
Other [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Discounted cash flow with inputs [Member] | Available-for-sale securities [Member] | Minimum [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Spread (bps)	3200.00%	23.00%
Other [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Discounted cash flow with inputs [Member] | Available-for-sale securities [Member] | Maximum [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Spread (bps)	31530.00%	205.10%
Other [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Discounted cash flow with inputs [Member] | Available-for-sale securities [Member] | Weighted Average [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Spread (bps)	9340.00%	76.10%
Other [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Other valuation technique [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	$ 29	$ 39
Subprime [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Other valuation technique [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	$ 2