FAIR VALUE MEASUREMENTS - Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023		Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022
Loans
Mortgage servicing rights	$ 2,785,614	$ 2,651,279	$ 2,122,196		$ 2,018,896	$ 1,790,980	$ 1,748,209
Derivative assets
Interest rate swaps	187,868		162,557	[1]
Liabilities
Participations issued	1,144,821		1,103,835	[1]
U.S. Treasury securities sold, not yet purchased	1,974,602		2,132,751
Derivative liabilities
Interest rate swaps	15,314		83,051
Residential mortgage loans
Loans
Residential mortgage loans	20,495,040		15,660,706
Fair Value, Measurements, Recurring
Assets
Agency mortgage-backed securities	64,390,905		66,308,788
Credit risk transfer securities	838,437		974,059
Loans
Mortgage servicing rights	2,785,614		2,122,196
Derivative assets
Other derivatives	171,044		136,213
Total assets	90,513,275		87,559,024
Liabilities
Debt issued by securitization vehicles	15,831,915		11,600,338
Participations issued	1,144,821		1,103,835
U.S. Treasury securities sold, not yet purchased	1,974,602		2,132,751
Derivative liabilities
Interest rate swaps	15,314		83,051
Other derivatives	85,515		219,244
Total liabilities	19,052,167		15,139,219
Fair Value, Measurements, Recurring | Assets transferred or pledged to securitization vehicles
Loans
Assets transferred or pledged to securitization vehicles	17,946,812		13,307,622
Fair Value, Measurements, Recurring | Non-Agency mortgage-backed securities
Assets
Mortgage-backed securities	1,702,859		2,108,274
Fair Value, Measurements, Recurring | Commercial mortgage-backed securities
Assets
Mortgage-backed securities	112,552		222,444
Fair Value, Measurements, Recurring | Residential mortgage loans
Loans
Residential mortgage loans	2,548,228		2,353,084
Fair Value, Measurements, Recurring | Interest rate swaps
Derivative assets
Interest rate swaps	16,824		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	1,723		0
Total assets	1,723		0
Liabilities
Debt issued by securitization vehicles	0		0
Participations issued	0		0
U.S. Treasury securities sold, not yet purchased	1,974,602		2,132,751
Derivative liabilities
Interest rate swaps	0		0
Other derivatives	81,730		179,835
Total liabilities	2,056,332		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	64,390,905		66,308,788
Credit risk transfer securities	838,437		974,059
Loans
Mortgage servicing rights	0		0
Derivative assets
Other derivatives	169,321		136,213
Total assets	87,725,938		85,436,828
Liabilities
Debt issued by securitization vehicles	15,831,915		11,600,338
Participations issued	1,144,821		1,103,835
U.S. Treasury securities sold, not yet purchased	0		0
Derivative liabilities
Interest rate swaps	15,314		83,051
Other derivatives	3,785		39,409
Total liabilities	16,995,835		12,826,633
Fair Value, Measurements, Recurring | Level 2 | Assets transferred or pledged to securitization vehicles
Loans
Assets transferred or pledged to securitization vehicles	17,946,812		13,307,622
Fair Value, Measurements, Recurring | Level 2 | Non-Agency mortgage-backed securities
Assets
Mortgage-backed securities	1,702,859		2,108,274
Fair Value, Measurements, Recurring | Level 2 | Commercial mortgage-backed securities
Assets
Mortgage-backed securities	112,552		222,444
Fair Value, Measurements, Recurring | Level 2 | Residential mortgage loans
Loans
Residential mortgage loans	2,548,228		2,353,084
Fair Value, Measurements, Recurring | Level 2 | Interest rate swaps
Derivative assets
Interest rate swaps	16,824		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	2,785,614		2,122,196
Derivative assets
Other derivatives	0		0
Total assets	2,785,614		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.