Fair Value Level 3 Valuation Inputs - Recurring (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at fair value	$ 7,825	$ 8,922
Net derivatives	231	401
Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	48,123	41,867
Available-for-sale securities	2,404	3,429
Mortgage loans held for investment, at fair value	7,825	8,922
Long-term debt	(27,567)	(30,579)
Recurring Fair Value Measurements [Member] | Alt-A and subprime private-label securities [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	629	1,306
Available-for-sale securities	57	592
Recurring Fair Value Measurements [Member] | Mortgage revenue bonds [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	315	434
Recurring Fair Value Measurements [Member] | Other [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	314	350
Fair Value, Inputs, Level 2 [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	8,576	6,332
Available-for-sale securities	1,612	2,477
Mortgage loans held for investment, at fair value	3,270,535	2,990,104
Mortgage loans held for sale, at lower of cost or fair value	229	238
Fair Value, Inputs, Level 2 [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	8,576	6,332
Available-for-sale securities	1,612	2,477
Mortgage loans held for investment, at fair value	7,137	7,985
Long-term debt	(27,094)	(30,027)
Fair Value, Inputs, Level 2 [Member] | Recurring Fair Value Measurements [Member] | Alt-A and subprime private-label securities [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	629	1,305
Available-for-sale securities	57	568
Fair Value, Inputs, Level 2 [Member] | Recurring Fair Value Measurements [Member] | Mortgage revenue bonds [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	0	0
Fair Value, Inputs, Level 2 [Member] | Recurring Fair Value Measurements [Member] | Other [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	8	8
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	274	91
Significant Unobservable Inputs (Level 3) [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	46	33
Available-for-sale securities	792	952
Mortgage loans held for investment, at fair value	127,650	216,404
Mortgage loans held for sale, at lower of cost or fair value	7,054	7,856
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	46	33
Available-for-sale securities	792	952
Mortgage loans held for investment, at fair value	688	937
Net derivatives	162	194
Long-term debt	(473)	(552)
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Agency [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	171
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Alt-A and subprime private-label securities [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	0	1
Available-for-sale securities	0	24
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Mortgage revenue bonds [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	315	434
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Other [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	306	342
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Consensus without inputs [Member] | Agency [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	107
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Discounted cash flow with inputs [Member] | Other [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	$ 267	$ 294
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Discounted cash flow with inputs [Member] | Other [Member] | Available-for-sale securities [Member] | Minimum [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Spread (bps)	3.00%	0.754%
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Discounted cash flow with inputs [Member] | Other [Member] | Available-for-sale securities [Member] | Maximum [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Spread (bps)	3.00%	3.90%
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Discounted cash flow with inputs [Member] | Other [Member] | Available-for-sale securities [Member] | Maximum [Member] | Measurement Input, Default Rate [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Debt Securities, Available-for-sale, Measurement Input		0.0470
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Discounted cash flow with inputs [Member] | Other [Member] | Available-for-sale securities [Member] | Maximum [Member] | Measurement Input, Constant Prepayment Rate [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Debt Securities, Available-for-sale, Measurement Input		0.082
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Discounted cash flow with inputs [Member] | Other [Member] | Available-for-sale securities [Member] | Maximum [Member] | Measurement Input, Loss Severity [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Debt Securities, Available-for-sale, Measurement Input		0.700
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Discounted cash flow with inputs [Member] | Other [Member] | Available-for-sale securities [Member] | Weighted Average [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Spread (bps)	3.00%	3.891%
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Discounted cash flow with inputs [Member] | Other [Member] | Available-for-sale securities [Member] | Weighted Average [Member] | Measurement Input, Default Rate [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Debt Securities, Available-for-sale, Measurement Input		0.0470
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Discounted cash flow with inputs [Member] | Other [Member] | Available-for-sale securities [Member] | Weighted Average [Member] | Measurement Input, Constant Prepayment Rate [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Debt Securities, Available-for-sale, Measurement Input		0.082
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Discounted cash flow with inputs [Member] | Other [Member] | Available-for-sale securities [Member] | Weighted Average [Member] | Measurement Input, Loss Severity [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Debt Securities, Available-for-sale, Measurement Input		0.700
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Single vendor with inputs [Member] | Mortgage revenue bonds [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	$ 222	$ 349
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Single vendor with inputs [Member] | Mortgage revenue bonds [Member] | Available-for-sale securities [Member] | Minimum [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Spread (bps)	0.23%	(0.005%)
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Single vendor with inputs [Member] | Mortgage revenue bonds [Member] | Available-for-sale securities [Member] | Maximum [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Spread (bps)	2.051%	3.328%
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Single vendor with inputs [Member] | Mortgage revenue bonds [Member] | Available-for-sale securities [Member] | Weighted Average [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Spread (bps)	0.761%	0.59%
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Other valuation technique [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Net derivatives	$ 15	$ 81
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Other valuation technique [Member] | Agency [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	46	32
Available-for-sale securities	64	152
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Other valuation technique [Member] | Alt-A and subprime private-label securities [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities		24
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Other valuation technique [Member] | Mortgage revenue bonds [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities		1
Available-for-sale securities	93	85
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Other valuation technique [Member] | Other [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	39	48
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Dealer Mark [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Net derivatives	147	113
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	1,076	1,750
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	471	631
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	605	1,119
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Nonrecurring [Member] | Single-Family [Member] | Internal model [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at fair value	555	818
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Nonrecurring [Member] | Multifamily [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at fair value	40	142
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Nonrecurring [Member] | Multifamily [Member] | Other valuation technique [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at fair value	$ 16	$ 40