Fair Value - Level 3 Valuation - Nonrecurring (Details) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, net of allowance for loan losses	$ 5,301,000,000	$ 6,490,000,000
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value Inputs, Quantitative Information [Line Items]
Total nonrecurring assets at fair value	166,256,000,000	227,635,000,000
Mortgage loans held-for-sale, at lower of cost or fair value	0	0
Mortgage loans held for investment, net of allowance for loan losses	0	0
Significant Other Observable Inputs (Level 2)
Fair Value Inputs, Quantitative Information [Line Items]
Total nonrecurring assets at fair value	3,850,387,000,000	3,578,668,000,000
Mortgage loans held-for-sale, at lower of cost or fair value	2,250,000,000	116,000,000
Mortgage loans held for investment, net of allowance for loan losses	3,745,286,000,000	3,512,672,000,000
Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Quantitative Information [Line Items]
Total nonrecurring assets at fair value	227,806,000,000	262,442,000,000
Mortgage loans held-for-sale, at lower of cost or fair value	6,129,000,000	5,502,000,000
Mortgage loans held for investment, net of allowance for loan losses	220,358,000,000	255,556,000,000
Nonrecurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value Inputs, Quantitative Information [Line Items]
Total nonrecurring assets at fair value	0	0
Nonrecurring | Significant Other Observable Inputs (Level 2)
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held-for-sale, at lower of cost or fair value	97,000,000	25,000,000
Total nonrecurring liabilities at fair value	0	0
Nonrecurring | Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Quantitative Information [Line Items]
Total nonrecurring assets at fair value	1,896,000,000	2,742,000,000
Mortgage loans held-for-sale, at lower of cost or fair value	829,000,000	1,087,000,000
Nonrecurring | Significant Unobservable Inputs (Level 3) | Consensus
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held-for-sale, at lower of cost or fair value	511,000,000	754,000,000
Nonrecurring | Significant Unobservable Inputs (Level 3) | Single Vendor
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held-for-sale, at lower of cost or fair value	316,000,000	333,000,000
Nonrecurring | Significant Unobservable Inputs (Level 3) | Various
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held-for-sale, at lower of cost or fair value	2,000,000	0
Nonrecurring | Significant Unobservable Inputs (Level 3) | Single-Family
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	164,000,000	261,000,000
Nonrecurring | Significant Unobservable Inputs (Level 3) | Single-Family | Various
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	9,000,000	11,000,000
Nonrecurring | Significant Unobservable Inputs (Level 3) | Single-Family | Accepted Offers
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	9,000,000	35,000,000
Nonrecurring | Significant Unobservable Inputs (Level 3) | Single-Family | Appraisals
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	54,000,000	89,000,000
Nonrecurring | Significant Unobservable Inputs (Level 3) | Single-Family | Internal Model
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, net of allowance for loan losses	592,000,000	979,000,000
Acquired property, net	67,000,000	41,000,000
Nonrecurring | Significant Unobservable Inputs (Level 3) | Single-Family | Walk Forwards
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	25,000,000	85,000,000
Nonrecurring | Significant Unobservable Inputs (Level 3) | Multifamily
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, net of allowance for loan losses	277,000,000	390,000,000
Nonrecurring | Significant Unobservable Inputs (Level 3) | Multifamily | Various
Fair Value Inputs, Quantitative Information [Line Items]
Acquired property, net	34,000,000	25,000,000
Nonrecurring | Significant Unobservable Inputs (Level 3) | Multifamily | Appraisals
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, net of allowance for loan losses	47,000,000	225,000,000
Nonrecurring | Significant Unobservable Inputs (Level 3) | Multifamily | Internal Model
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, net of allowance for loan losses	113,000,000	125,000,000
Nonrecurring | Significant Unobservable Inputs (Level 3) | Multifamily | Broker Price Opinion
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, net of allowance for loan losses	$ 117,000,000	$ 40,000,000