DEBT OBLIGATIONS, NET - Financing Strategy in Current Market Conditions (Details) - USD ($)			3 Months Ended		12 Months Ended
		Apr. 27, 2020	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2020	Apr. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Amount paid down, FHLB Borrowings			$ 38,000,000.0	$ 646,800,000	$ 446,900,000
Debt obligations, net	[1]		4,209,864,000		4,209,864,000			$ 4,859,873,000
Loans financed		$ 481,300,000
Advance rate		64.50%
Cash, cash equivalents and restricted cash			1,284,284,000		$ 1,284,284,000			$ 355,746,000	$ 98,450,000	$ 182,683,000
Prepayment penalties				$ 6,500,000
Senior Notes Due 2027 | Senior Unsecured Notes
Debt Instrument [Line Items]
Amount paid down			$ 257,700,000
Non-Recourse Notes | CLO Debt
Debt Instrument [Line Items]
Debt obligations, net		$ 310,200,000
Non-Recourse Notes | Secured Debt | Koch Real Estate Investments, LLC
Debt Instrument [Line Items]
Non-mark to market financing facility						$ 206,400,000
Maturing February 11 2021 | Revolving Credit Facility
Debt Instrument [Line Items]
Committed financing							$ 266,400,000

[1] Includes amounts relating to consolidated variable interest entities. See Note 1 and Note 10.