AVAILABLE-FOR-SALE SECURITIES - AFS Securities by Collateral Type (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
AFS investment securities	$ 0	$ 1,290,825,648
Available for sale securities	$ 0	1,290,825,648
Agency
Debt Securities, Available-for-sale [Line Items]
AFS investment securities		1,285,083,648
Agency | Federal Home Loan Mortgage Corporation
Debt Securities, Available-for-sale [Line Items]
AFS investment securities		530,640,091
Agency | Federal National Mortgage Association
Debt Securities, Available-for-sale [Line Items]
AFS investment securities		754,443,557
Multi-Family
Debt Securities, Available-for-sale [Line Items]
AFS investment securities		$ 5,742,000