INVESTMENTS IN MORTGAGE SERVICING RIGHTS AND MORTGAGE SERVICING RIGHTS FINANCING RECEIVABLES - Schedule of Mortgage Rights Acquired (Details) - USD ($) $ in Millions														12 Months Ended
	Dec. 31, 2018	Nov. 08, 2018	Sep. 28, 2018	Aug. 31, 2018	Jun. 28, 2018	Jun. 04, 2018	Jun. 01, 2018	May 31, 2018	May 25, 2018	May 01, 2018	Mar. 28, 2018	Feb. 28, 2018	Jan. 16, 2018	Dec. 31, 2018
Schedule of Investments in Mortgage Servicing Rights [Line Items]
Unpaid Principal Balance	$ 103,000.0													$ 103,000.0
Payments to acquire mortgage servicing rights (MSR)		$ 1.5		$ 220.5	$ 66.5	$ 19.3	$ 6.1	$ 79.9	$ 26.3	$ 36.2	$ 96.6	$ 33.5		1,186.0
MSRs
Schedule of Investments in Mortgage Servicing Rights [Line Items]
Unpaid Principal Balance		$ 100.0		$ 18,500.0	$ 4,700.0	$ 2,100.0	$ 500.0	$ 6,100.0	$ 2,100.0	$ 4,600.0	$ 8,100.0	$ 3,300.0
Batch 1
Schedule of Investments in Mortgage Servicing Rights [Line Items]
Payments to acquire mortgage servicing rights (MSR)	81.4		$ 13.6										$ 101.5
Batch 1 | MSRs
Schedule of Investments in Mortgage Servicing Rights [Line Items]
Unpaid Principal Balance	7,000.0		1,100.0										11,500.0	7,000.0
Batch 2
Schedule of Investments in Mortgage Servicing Rights [Line Items]
Payments to acquire mortgage servicing rights (MSR)	135.7		126.4										81.0
Batch 2 | MSRs
Schedule of Investments in Mortgage Servicing Rights [Line Items]
Unpaid Principal Balance	9,800.0		$ 10,100.0										$ 7,800.0	9,800.0
Various Acquisition
Schedule of Investments in Mortgage Servicing Rights [Line Items]
Payments to acquire mortgage servicing rights (MSR)														60.0
Various Acquisition | MSRs
Schedule of Investments in Mortgage Servicing Rights [Line Items]
Unpaid Principal Balance	$ 5,600.0													$ 5,600.0