FAIR VALUE MEASUREMENTS - Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023		Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Loans
Mortgage servicing rights	$ 2,693,057	$ 2,785,614	$ 2,122,196		$ 2,234,813	$ 2,018,896	$ 1,748,209
Derivative assets
Interest rate swaps	59,071		162,557	[1]
Liabilities
Participations issued	467,006		1,103,835	[1]
U.S. Treasury securities sold, not yet purchased	2,043,519		2,132,751	[1]
Derivative liabilities
Interest rate swaps	59,297		83,051
Residential mortgage loans
Loans
Residential mortgage loans	23,349,620		15,660,706
Fair Value, Measurements, Recurring
Assets
Agency mortgage-backed securities	69,150,399		66,308,788
Credit risk transfer securities	826,841		974,059
Loans
Mortgage servicing rights	2,693,057		2,122,196
Derivative assets
Other derivatives	55,977		136,213
Total assets	97,801,925		87,559,024
Liabilities
Debt issued by securitization vehicles	18,709,118		11,600,338
Participations issued	467,006		1,103,835
U.S. Treasury securities sold, not yet purchased	2,043,519		2,132,751
Derivative liabilities
Interest rate swaps	59,297		83,051
Other derivatives	43,331		219,244
Total liabilities	21,322,271		15,139,219
Fair Value, Measurements, Recurring | Assets transferred or pledged to securitization vehicles
Loans
Assets transferred or pledged to securitization vehicles	21,044,007		13,307,622
Fair Value, Measurements, Recurring | Non-Agency mortgage-backed securities
Assets
Mortgage-backed securities	1,616,696		2,108,274
Fair Value, Measurements, Recurring | Commercial mortgage-backed securities
Assets
Mortgage-backed securities	106,241		222,444
Fair Value, Measurements, Recurring | Residential mortgage loans
Loans
Residential mortgage loans	2,305,613		2,353,084
Fair Value, Measurements, Recurring | Interest rate swaps
Derivative assets
Interest rate swaps	3,094		26,344
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
Other derivatives	23,256		0
Total assets	23,256		0
Liabilities
Debt issued by securitization vehicles	0		0
Participations issued	0		0
U.S. Treasury securities sold, not yet purchased	2,043,519		2,132,751
Derivative liabilities
Interest rate swaps	0		0
Other derivatives	32,117		179,835
Total liabilities	2,075,636		2,312,586
Fair Value, Measurements, Recurring | Level 1 | Assets transferred or pledged to securitization vehicles
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 1 | Interest rate swaps
Derivative assets
Interest rate swaps	0		0
Fair Value, Measurements, Recurring | Level 2
Assets
Agency mortgage-backed securities	69,150,399		66,308,788
Credit risk transfer securities	826,841		974,059
Loans
Mortgage servicing rights	0		0
Derivative assets
Other derivatives	32,721		136,213
Total assets	95,085,612		85,436,828
Liabilities
Debt issued by securitization vehicles	18,709,118		11,600,338
Participations issued	467,006		1,103,835
U.S. Treasury securities sold, not yet purchased	0		0
Derivative liabilities
Interest rate swaps	59,297		83,051
Other derivatives	11,214		39,409
Total liabilities	19,246,635		12,826,633
Fair Value, Measurements, Recurring | Level 2 | Assets transferred or pledged to securitization vehicles
Loans
Assets transferred or pledged to securitization vehicles	21,044,007		13,307,622
Fair Value, Measurements, Recurring | Level 2 | Non-Agency mortgage-backed securities
Assets
Mortgage-backed securities	1,616,696		2,108,274
Fair Value, Measurements, Recurring | Level 2 | Commercial mortgage-backed securities
Assets
Mortgage-backed securities	106,241		222,444
Fair Value, Measurements, Recurring | Level 2 | Residential mortgage loans
Loans
Residential mortgage loans	2,305,613		2,353,084
Fair Value, Measurements, Recurring | Level 2 | Interest rate swaps
Derivative assets
Interest rate swaps	3,094		26,344
Fair Value, Measurements, Recurring | Level 3
Assets
Agency mortgage-backed securities	0		0
Credit risk transfer securities	0		0
Loans
Mortgage servicing rights	2,693,057		2,122,196
Derivative assets
Other derivatives	0		0
Total assets	2,693,057		2,122,196
Liabilities
Debt issued by securitization vehicles	0		0
Participations issued	0		0
U.S. Treasury securities sold, not yet purchased	0		0
Derivative liabilities
Interest rate swaps	0		0
Other derivatives	0		0
Total liabilities	0		0
Fair Value, Measurements, Recurring | Level 3 | Assets transferred or pledged to securitization vehicles
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
Fair Value, Measurements, Recurring | Level 3 | Interest rate swaps
Derivative assets
Interest rate swaps	$ 0		$ 0

[1] (1)Derived from the audited consolidated financial statements at December 31, 2023.