FAIR VALUE MEASUREMENTS - Fair Value Liabilities Measured at Fair Value on a Recurring Basis using Level 3 Inputs (Details) - Level 3 - Recurring Basis - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, liability, beginning balance	$ 1,695,519	$ 746,737
Gains (losses) included in net income
Included in servicing revenue, net	(3,538)	(14,164)
Included in other income	(11,954)	5,810
Interest income	0
Purchases, sales and payments
Purchases	0	1,520,382
Proceeds from sales	(15,378)
Payments	(1,149,087)	(562,588)
Other	496	(658)
Fair value, liability, ending balance	516,058	1,695,519
Excess Spread Financing
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, liability, beginning balance	18,420	31,777
Gains (losses) included in net income
Included in servicing revenue, net	(3,538)	(14,164)
Included in other income	0	0
Interest income	0
Purchases, sales and payments
Purchases	0	0
Proceeds from sales	(15,378)
Payments	0	0
Other	496	807
Fair value, liability, ending balance	0	18,420
Mortgage-Backed Securities Issued
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, liability, beginning balance	1,662,852	659,738
Gains (losses) included in net income
Included in servicing revenue, net	0	0
Included in other income	(12,991)	966
Interest income	0
Purchases, sales and payments
Purchases	0	1,520,382
Proceeds from sales	0
Payments	(1,138,754)	(516,769)
Other	0	(1,465)
Fair value, liability, ending balance	511,107	1,662,852
Contingent Consideration
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, liability, beginning balance	14,247	55,222
Gains (losses) included in net income
Included in servicing revenue, net	0	0
Included in other income	1,037	4,844
Interest income	0
Purchases, sales and payments
Purchases	0	0
Proceeds from sales	0
Payments	(10,333)	(45,819)
Other	0	0
Fair value, liability, ending balance	$ 4,951	$ 14,247