FAIR VALUE MEASUREMENTS - Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Loans
Mortgage servicing rights	$ 2,122,196	$ 1,748,209	$ 544,562
Liabilities
Participations issued	1,103,835	800,849
U.S. Treasury securities sold, not yet purchased	2,132,751	0
Residential mortgage loans
Loans
Residential mortgage loans	15,660,706	10,931,744
Fair Value, Measurements, Recurring
Assets
Agency mortgage-backed securities	66,308,788	62,274,895
Credit risk transfer securities	974,059	997,557
Loans
Mortgage servicing rights	2,122,196	1,748,209
Derivative assets
Interest rate swaps	26,344	33,006
Other derivatives	136,213	309,058
Total assets	87,559,024	78,811,924
Liabilities
Debt issued by securitization vehicles	11,600,338	7,744,160
Participations issued	1,103,835	800,849
U.S. Treasury securities sold, not yet purchased	2,132,751
Derivative liabilities
Interest rate swaps	83,051	108,724
Other derivatives	219,244	95,448
Total liabilities	15,139,219	8,749,181
Fair Value, Measurements, Recurring \| Asset Transferred or Pledged
Loans
Assets transferred or pledged to securitization vehicles	13,307,622	9,121,912
Fair Value, Measurements, Recurring \| Non-Agency mortgage-backed securities
Assets
Mortgage-backed securities	2,108,274	1,991,146
Fair Value, Measurements, Recurring \| Commercial mortgage-backed securities
Assets
Mortgage-backed securities	222,444	526,309
Fair Value, Measurements, Recurring \| Residential mortgage loans
Loans
Residential mortgage loans	2,353,084	1,809,832
Fair Value, Measurements, Recurring \| Level 1
Assets
Agency mortgage-backed securities	0	0
Credit risk transfer securities	0	0
Loans
Mortgage servicing rights	0	0
Derivative assets
Interest rate swaps	0	0
Other derivatives	0	33,179
Total assets	0	33,179
Liabilities
Debt issued by securitization vehicles	0	0
Participations issued	0	0
U.S. Treasury securities sold, not yet purchased	2,132,751
Derivative liabilities
Interest rate swaps	0	0
Other derivatives	179,835	11,919
Total liabilities	2,312,586	11,919
Fair Value, Measurements, Recurring \| Level 1 \| Asset Transferred or Pledged
Loans
Assets transferred or pledged to securitization vehicles	0	0
Fair Value, Measurements, Recurring \| Level 1 \| Non-Agency mortgage-backed securities
Assets
Mortgage-backed securities	0	0
Fair Value, Measurements, Recurring \| Level 1 \| Commercial mortgage-backed securities
Assets
Mortgage-backed securities	0	0
Fair Value, Measurements, Recurring \| Level 1 \| Residential mortgage loans
Loans
Residential mortgage loans	0	0
Fair Value, Measurements, Recurring \| Level 2
Assets
Agency mortgage-backed securities	66,308,788	62,274,895
Credit risk transfer securities	974,059	997,557
Loans
Mortgage servicing rights	0	0
Derivative assets
Interest rate swaps	26,344	33,006
Other derivatives	136,213	275,879
Total assets	85,436,828	77,030,536
Liabilities
Debt issued by securitization vehicles	11,600,338	7,744,160
Participations issued	1,103,835	800,849
U.S. Treasury securities sold, not yet purchased	0
Derivative liabilities
Interest rate swaps	83,051	108,724
Other derivatives	39,409	83,529
Total liabilities	12,826,633	8,737,262
Fair Value, Measurements, Recurring \| Level 2 \| Asset Transferred or Pledged
Loans
Assets transferred or pledged to securitization vehicles	13,307,622	9,121,912
Fair Value, Measurements, Recurring \| Level 2 \| Non-Agency mortgage-backed securities
Assets
Mortgage-backed securities	2,108,274	1,991,146
Fair Value, Measurements, Recurring \| Level 2 \| Commercial mortgage-backed securities
Assets
Mortgage-backed securities	222,444	526,309
Fair Value, Measurements, Recurring \| Level 2 \| Residential mortgage loans
Loans
Residential mortgage loans	2,353,084	1,809,832
Fair Value, Measurements, Recurring \| Level 3
Assets
Agency mortgage-backed securities	0	0
Credit risk transfer securities	0	0
Loans
Mortgage servicing rights	2,122,196	1,748,209
Derivative assets
Interest rate swaps	0	0
Other derivatives	0	0
Total assets	2,122,196	1,748,209
Liabilities
Debt issued by securitization vehicles	0	0
Participations issued	0	0
U.S. Treasury securities sold, not yet purchased	0
Derivative liabilities
Interest rate swaps	0	0
Other derivatives	0	0
Total liabilities	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Asset Transferred or Pledged
Loans
Assets transferred or pledged to securitization vehicles	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Non-Agency mortgage-backed securities
Assets
Mortgage-backed securities	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Commercial mortgage-backed securities
Assets
Mortgage-backed securities	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Residential mortgage loans
Loans
Residential mortgage loans	$ 0	$ 0

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