REPURCHASE AGREEMENTS AND OTHER BORROWINGS - Remaining Maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 3,818,590	$ 3,726,523
Repurchase agreement liabilities associated with these liabilities	$ 3,619,280	$ 3,533,786
Net weighted average borrowing rate	1.86%	1.54%
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	$ 2,263,087	$ 1,983,958
Repurchase agreement liabilities associated with these liabilities	$ 2,127,683	$ 1,871,833
Net weighted average borrowing rate	1.91%	1.53%
Between 31 and 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 1,096,463	$ 1,266,590
Repurchase agreement liabilities associated with these liabilities	$ 1,038,162	$ 1,208,518
Net weighted average borrowing rate	1.88%	1.53%
Greater Than 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 459,040	$ 475,975
Repurchase agreement liabilities associated with these liabilities	$ 453,435	$ 453,435
Net weighted average borrowing rate	1.57%	1.57%