SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of General and Administrative Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Other Income [Line Items]
Legal and professional expense	$ 89,489	$ 45,234	$ 40,182
Loan origination expense	59,418	16,050	0
Occupancy expense	19,388
Occupancy expense		8,868	0
Other	84,251	51,557	26,793
General and Administrative Expense	538,035	231,579	67,159
Servicing
Schedule Of Other Income [Line Items]
Compensation and benefits expense, servicing	121,004	47,084	184
Origination
Schedule Of Other Income [Line Items]
Compensation and benefits expense, servicing	$ 164,485	$ 62,786	$ 0