LOANS - Investment Loan Activity (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
Mortgage Loans on Real Estate [Line Items]
Beginning balance January 1, 2019	$ 4,585,975	[1],[2]
Purchases	716,097
Sales and transfers	(1,357,997)
Principal Payments	(66,755)
Gains / (losses)	(1,243)
Amortization / accretion	3,247
Mortgage Loans on Real Estate	3,879,324	[2]
Residential
Mortgage Loans on Real Estate [Line Items]
Beginning balance January 1, 2019	1,359,806
Purchases	425,488
Sales and transfers	(444,963)
Principal Payments	(32,676)
Gains / (losses)	4,460
Amortization / accretion	(395)
Mortgage Loans on Real Estate	1,311,720
Commercial
Mortgage Loans on Real Estate [Line Items]
Beginning balance January 1, 2019	1,338,987
Purchases	165,258
Sales and transfers	(733,922)
Principal Payments	(534)
Gains / (losses)	(5,703)
Amortization / accretion	911
Mortgage Loans on Real Estate	764,997
Corporate
Mortgage Loans on Real Estate [Line Items]
Beginning balance January 1, 2019	1,887,182
Purchases	125,351
Sales and transfers	(179,112)
Principal Payments	(33,545)
Gains / (losses)	0
Amortization / accretion	2,731
Mortgage Loans on Real Estate	$ 1,802,607

[1] (1) Derived from the audited consolidated financial statements at December 31, 2018.

[2] (4) Includes $101.3 million and $97.5 million of residential mortgage loans held for sale, $42.0 million and $42.2 million of commercial mortgage loans held for sale and $44.5 million and $0 of corporate loans held for sale at March 31, 2019 and December 31, 2018, respectively.