VARIABLE INTEREST ENTITIES - Statement of Financial Condition of VIEs Reflected in Consolidated Statements of Financial Condition (Details) - USD ($) $ in Thousands		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015	Dec. 31, 2014
Assets
Cash and cash equivalents		$ 706,589	[1],[2]	$ 1,539,746	[2]	$ 1,769,258	$ 1,741,244
Commercial real estate debt investments	[3]	3,089,108	[1]	4,321,739
Residential mortgages loans	[4]	1,438,322	[1]	342,289
Mortgage servicing rights		580,860	[1]	652,216
Accrued interest receivable		323,526	[1]	270,400
Other derivatives, at fair value		283,613	[1]	171,266
Other assets		384,117	[1]	333,063
Total assets		101,760,050	[1]	87,905,046
Liabilities
Other secured financing		3,837,528	[1]	3,884,708
Other derivatives, at fair value		38,725	[1]	86,437
Accrued interest payable		253,068	[1]	163,013
Accounts payable and other liabilities		207,770	[1]	184,319
Total liabilities		86,888,477	[1]	75,329,074
Consolidated VIEs
Assets
Cash and cash equivalents		42,300		23,200
Residential mortgages loans		478,800		165,900
Consolidated VIEs | FREMF Trusts
Assets
Cash and cash equivalents		0		0
Commercial real estate debt investments		2,826,357		3,890,807
Residential mortgages loans		0		0
Mortgage servicing rights		0		0
Accrued interest receivable		10,339		8,690
Other derivatives, at fair value		0		0
Other assets		0		138
Total assets		2,836,696		3,899,635
Liabilities
Securitized debt (non-recourse) at fair value		2,620,952		3,609,164
Other secured financing		0		0
Other derivatives, at fair value				0
Accrued interest payable		4,554		4,350
Accounts payable and other liabilities		0		0
Total liabilities		2,625,506		3,613,514
Consolidated VIEs | Residential Mortgage Loan Trusts
Assets
Cash and cash equivalents		0		0
Commercial real estate debt investments		0		0
Residential mortgages loans		478,811		165,869
Mortgage servicing rights		0		0
Accrued interest receivable		1,599		836
Other derivatives, at fair value		0		0
Other assets		1,418		0
Total assets		481,828		166,705
Liabilities
Securitized debt (non-recourse) at fair value		350,819		46,638
Other secured financing		0		0
Other derivatives, at fair value				0
Accrued interest payable		931		107
Accounts payable and other liabilities		112		662
Total liabilities		351,862		47,407
Consolidated VIEs | MSR Silo
Assets
Cash and cash equivalents		42,293		23,198
Commercial real estate debt investments		0		0
Residential mortgages loans		19,667		8,309
Mortgage servicing rights		580,860		652,216
Accrued interest receivable		0		0
Other derivatives, at fair value		1		9
Other assets		32,354		35,540
Total assets		675,175		719,272
Liabilities
Securitized debt (non-recourse) at fair value		0		0
Other secured financing		10,496		3,825
Other derivatives, at fair value				9
Accrued interest payable		0		0
Accounts payable and other liabilities		4,856		14,007
Total liabilities		$ 15,352		$ 17,841

[1] As a result of a change to a clearing organization’s rulebook effective January 3, 2017, beginning with the first quarter 2017 and in subsequent periods the Company is presenting the fair value of centrally cleared interest rate swaps net of variation margin pledged under such transactions. The variation margin was previously reported under cash and cash equivalents and is currently reported as a reduction to interest rate swaps, at fair value. Balances reported prior to the effective date will not be adjusted.

[2] Includes cash of consolidated VIEs of $42.3 million and $23.2 million at December 31, 2017 and 2016, respectively.

[3] Includes senior securitized commercial mortgage loans of consolidated VIEs carried at fair value of $2.8 billion and $3.9 billion at December 31, 2017 and 2016, respectively.

[4] Includes securitized residential mortgage loans of a consolidated VIE carried at fair value of $478.8 million and $165.9 million at December 31, 2017 and 2016, respectively