USE OF DERIVATIVES, OFFSETTING DISCLOSURES AND CHANGES IN OTHER COMPREHENSIVE INCOME BY COMPONENT - Changes in Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 1,247,687	$ 1,298,324	$ 1,390,771
Other comprehensive income (loss)	(43,773)	(30,309)	(91,327)
Ending Balance	1,238,876	1,247,687	1,298,324
Gains and Losses on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	151	(18,434)	(25,309)
Other comprehensive income (loss) before reclassifications	21,426	(1,370)	(21,675)
Amounts reclassified from accumulated other comprehensive income	(4,808)	19,955	28,550
Cumulative effect adjustment	105
Other comprehensive income (loss)	16,723	18,585	6,875
Ending Balance	16,874	151	(18,434)
Unrealized Gains and Losses on Available-for-sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	105,635	154,529	252,731
Other comprehensive income (loss) before reclassifications	(60,391)	(48,894)	(98,202)
Amounts reclassified from accumulated other comprehensive income	0	0	0
Cumulative effect adjustment	0
Other comprehensive income (loss)	(60,391)	(48,894)	(98,202)
Ending Balance	45,244	105,635	154,529
Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	105,786	136,095	227,422
Other comprehensive income (loss) before reclassifications	(38,965)	(50,264)	(119,877)
Amounts reclassified from accumulated other comprehensive income	(4,808)	19,955	28,550
Cumulative effect adjustment	105
Other comprehensive income (loss)	(43,668)	(30,309)	(91,327)
Ending Balance	$ 62,118	$ 105,786	$ 136,095