REPURCHASE AGREEMENTS AND OTHER BORROWINGS - Remaining Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 3,726,523	$ 2,970,880
Repurchase agreement liabilities associated with these liabilities	$ 3,533,786	$ 2,793,705
Net weighted average borrowing rate	1.54%	1.00%
Overnight (1 Day or Less) [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 0	$ 0
Repurchase agreement liabilities associated with these liabilities	$ 0	$ 0
Net weighted average borrowing rate	0.00%	0.00%
Between 2 and 30 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 1,983,958	$ 2,284,815
Repurchase agreement liabilities associated with these liabilities	$ 1,871,833	$ 2,154,766
Net weighted average borrowing rate	1.53%	1.01%
Between 31 and 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 1,266,590	$ 686,065
Repurchase agreement liabilities associated with these liabilities	$ 1,208,518	$ 638,939
Net weighted average borrowing rate	1.53%	0.96%
Greater Than 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 475,975	$ 0
Repurchase agreement liabilities associated with these liabilities	$ 453,435	$ 0
Net weighted average borrowing rate	1.57%	0.00%