Fair Value - Significant Unobservable Inputs (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) Item	Dec. 31, 2020 USD ($) $ / sqft Item
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities	$ 148,583	$ 167,349
Domestic servicing rights	15,942	13,202
Assets	81,980,864	80,873,509
Liabilities	77,133,752	76,010,933
Fair value measurements on recurring basis | Loans held-for-sale
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Loans under fair value option	2,190,765	1,022,979
Fair value measurements on recurring basis | RMBS
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities	148,583	167,349
Fair value measurements on recurring basis | CMBS
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities	23,503	19,457
Fair value measurements on recurring basis | Domestic servicing rights, at fair value
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Domestic servicing rights	15,942	13,202
Fair value measurements on recurring basis | Level III | Loans held-for-sale
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Loans under fair value option	$ 1,806,216	$ 1,022,979
Fair value measurements on recurring basis | Level III | Loans held-for-sale | Minimum
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Loans held-for-sale, duration	5 years	7 years 3 months 18 days
Fair value measurements on recurring basis | Level III | Loans held-for-sale | Minimum | Debt yield
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Loans held-for-sale, measurement input	0.033	0.033
Fair value measurements on recurring basis | Level III | Loans held-for-sale | Minimum | FICO score
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Loans held-for-sale, measurement input | Item	582	519
Fair value measurements on recurring basis | Level III | Loans held-for-sale | Minimum | LTV
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Loans held-for-sale, measurement input	0.07	0.05
Fair value measurements on recurring basis | Level III | Loans held-for-sale | Minimum | Purchase price
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Loans held-for-sale, measurement input	0.800	0.844
Fair value measurements on recurring basis | Level III | Loans held-for-sale | Maximum
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Loans held-for-sale, duration	39 years 10 months 24 days	39 years 3 months 18 days
Fair value measurements on recurring basis | Level III | Loans held-for-sale | Maximum | Debt yield
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Loans held-for-sale, measurement input	0.061	0.097
Fair value measurements on recurring basis | Level III | Loans held-for-sale | Maximum | FICO score
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Loans held-for-sale, measurement input | Item	823	823
Fair value measurements on recurring basis | Level III | Loans held-for-sale | Maximum | LTV
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Loans held-for-sale, measurement input	0.94	0.94
Fair value measurements on recurring basis | Level III | Loans held-for-sale | Maximum | Purchase price
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Loans held-for-sale, measurement input	1.068	1.048
Fair value measurements on recurring basis | Level III | Loans held-for-sale | Weighted-average
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Loans held-for-sale, duration	29 years	26 years 3 months 18 days
Fair value measurements on recurring basis | Level III | Loans held-for-sale | Weighted-average | Debt yield
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Loans held-for-sale, measurement input	0.044	0.059
Fair value measurements on recurring basis | Level III | Loans held-for-sale | Weighted-average | FICO score
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Loans held-for-sale, measurement input	746	727
Fair value measurements on recurring basis | Level III | Loans held-for-sale | Weighted-average | LTV
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Loans held-for-sale, measurement input	0.67	0.68
Fair value measurements on recurring basis | Level III | Loans held-for-sale | Weighted-average | Purchase price
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Loans held-for-sale, measurement input	0.994	0.998
Fair value measurements on recurring basis | Level III | RMBS
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities	$ 148,583	$ 167,349
Loss severity for specified percentage of portfolio (as a percent)	9.00%	23.00%
Fair value measurements on recurring basis | Level III | RMBS | Minimum
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Loss severity for specified percentage of portfolio (as a percent)	45.00%
Fair value measurements on recurring basis | Level III | RMBS | Minimum | Constant prepayment rate
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0.041	0.036
Fair value measurements on recurring basis | Level III | RMBS | Minimum | Constant default rate
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0.005	0.007
Fair value measurements on recurring basis | Level III | RMBS | Minimum | Loss severity
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0	0
Fair value measurements on recurring basis | Level III | RMBS | Minimum | Delinquency rate
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0.07	0.10
Fair value measurements on recurring basis | Level III | RMBS | Minimum | Servicer advances
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0.21	0.23
Fair value measurements on recurring basis | Level III | RMBS | Minimum | Annual coupon deterioration
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0	0
Fair value measurements on recurring basis | Level III | RMBS | Minimum | Putback amount per projected total collateral loss
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0	0
Fair value measurements on recurring basis | Level III | RMBS | Maximum
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Loss severity for specified percentage of portfolio (as a percent)	80.00%
Fair value measurements on recurring basis | Level III | RMBS | Maximum | Constant prepayment rate
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0.173	0.194
Fair value measurements on recurring basis | Level III | RMBS | Maximum | Constant default rate
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0.052	0.054
Fair value measurements on recurring basis | Level III | RMBS | Maximum | Loss severity
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0.83	0.85
Fair value measurements on recurring basis | Level III | RMBS | Maximum | Delinquency rate
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0.35	0.32
Fair value measurements on recurring basis | Level III | RMBS | Maximum | Servicer advances
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0.83	0.82
Fair value measurements on recurring basis | Level III | RMBS | Maximum | Annual coupon deterioration
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0.015	0.009
Fair value measurements on recurring basis | Level III | RMBS | Maximum | Putback amount per projected total collateral loss
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0.08	0.17
Fair value measurements on recurring basis | Level III | RMBS | Weighted-average | Constant prepayment rate
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0.088	0.076
Fair value measurements on recurring basis | Level III | RMBS | Weighted-average | Constant default rate
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0.022	0.024
Fair value measurements on recurring basis | Level III | RMBS | Weighted-average | Loss severity
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0.11	0.20
Fair value measurements on recurring basis | Level III | RMBS | Weighted-average | Delinquency rate
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0.19	0.19
Fair value measurements on recurring basis | Level III | RMBS | Weighted-average | Servicer advances
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0.52	0.54
Fair value measurements on recurring basis | Level III | RMBS | Weighted-average | Annual coupon deterioration
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0.001	0.001
Fair value measurements on recurring basis | Level III | RMBS | Weighted-average | Putback amount per projected total collateral loss
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0.004	0.008
Fair value measurements on recurring basis | Level III | CMBS
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities	$ 23,503	$ 19,457
Fair value measurements on recurring basis | Level III | CMBS | Minimum
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale debt securities, term	0 days	0 years
Fair value measurements on recurring basis | Level III | CMBS | Minimum | Debt yield
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0	0
Fair value measurements on recurring basis | Level III | CMBS | Maximum
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale debt securities, term	8 years	7 years 7 months 6 days
Fair value measurements on recurring basis | Level III | CMBS | Maximum | Debt yield
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	3.013	5.366
Fair value measurements on recurring basis | Level III | CMBS | Weighted-average
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale debt securities, term	5 years 3 months 18 days	5 years 3 months 18 days
Fair value measurements on recurring basis | Level III | CMBS | Weighted-average | Debt yield
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Available-for-sale securities, measurement input	0.085	0.071
Fair value measurements on recurring basis | Level III | Domestic servicing rights, at fair value
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Domestic servicing rights	$ 15,942	$ 13,202
Fair value measurements on recurring basis | Level III | Domestic servicing rights, at fair value | Debt yield
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Servicing asset, measurement input	0.0725	0.0750
Fair value measurements on recurring basis | Level III | Domestic servicing rights, at fair value | Discount rate
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Servicing asset, measurement input	0.15	0.15
Fair value measurements on recurring basis | Level III | Domestic servicing rights, at fair value | Weighted-average | Debt yield
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Servicing asset, measurement input	0.0725	0.0750
Fair value measurements on recurring basis | Level III | Domestic servicing rights, at fair value | Weighted-average | Discount rate
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Servicing asset, measurement input	0.15	0.15
Fair value measurements on recurring basis | Level III | VIE assets | Minimum
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
VIE duration (in years)	0 days	0 years
Fair value measurements on recurring basis | Level III | VIE assets | Minimum | Debt yield
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
VIE assets, measurement input	0	0
Fair value measurements on recurring basis | Level III | VIE assets | Maximum
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
VIE duration (in years)	19 years 1 month 6 days	16 years 3 months 18 days
Fair value measurements on recurring basis | Level III | VIE assets | Maximum | Debt yield
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
VIE assets, measurement input	3.407	3.122
Fair value measurements on recurring basis | Level III | VIE assets | Weighted-average
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
VIE duration (in years)	3 years 3 months 18 days	3 years 9 months 18 days
Fair value measurements on recurring basis | Level III | VIE assets | Weighted-average | Debt yield
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
VIE assets, measurement input	0.121	0.143
Fair value measurements on recurring basis | Level III | VIE liabilities | Minimum
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
VIE duration (in years)	0 days	0 years
Fair value measurements on recurring basis | Level III | VIE liabilities | Minimum | Debt yield
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
VIE liabilities, measurement input	0	0
Fair value measurements on recurring basis | Level III | VIE liabilities | Maximum
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
VIE duration (in years)	13 years 4 months 24 days	10 years 9 months 18 days
Fair value measurements on recurring basis | Level III | VIE liabilities | Maximum | Debt yield
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
VIE liabilities, measurement input	3.407	3.122
Fair value measurements on recurring basis | Level III | VIE liabilities | Weighted-average
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
VIE duration (in years)	2 years 7 months 6 days	3 years 9 months 18 days
Fair value measurements on recurring basis | Level III | VIE liabilities | Weighted-average | Debt yield
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
VIE liabilities, measurement input	0.063	0.144
Primary beneficiary | Fair value measurements on recurring basis | VIE assets
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Assets	$ 62,346,480	$ 64,238,328
Primary beneficiary | Fair value measurements on recurring basis | VIE liabilities
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Liabilities	60,894,975	62,776,371
Primary beneficiary | Fair value measurements on recurring basis | Level III | VIE assets
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Assets	62,346,480	64,238,328
Primary beneficiary | Fair value measurements on recurring basis | Level III | VIE liabilities
Quantitative information for Level 3 Fair Value Measurements for assets and liabilities measured at fair value on recurring basis
Liabilities	$ 4,530,589	$ 2,019,876