AVAILABLE-FOR-SALE SECURITIES - Fair Value of AFS Investment Securities by Rate Type (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
AFS debt securities	$ 0	$ 1,290,825,648
Available-for-sale securities	$ 0	1,290,825,648
Adjustable rate
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities		1,284,237,670
Fixed rate
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities		6,587,978
Agency
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities		1,285,083,648
Agency | Adjustable rate
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities		1,284,237,670
Agency | Fixed rate
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities		845,978
Non-Agency
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities		0
Non-Agency | Adjustable rate
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities		0
Non-Agency | Fixed rate
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities		0
Multi-Family
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities		5,742,000
Multi-Family | Adjustable rate
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities		0
Multi-Family | Fixed rate
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities		$ 5,742,000