FEDERAL HOME LOAN BANK ADVANCES - SHORT-TERM FHLB ADVANCES (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
FHLB advances
Federal Home Loan Bank advances	$ 862,500		$ 862,500		$ 699,500
Interest rate (as percent)	1.18%		1.18%
Federal Home Loan Bank overnight advances
FHLB advances
Federal Home Loan Bank advances	$ 430,000		$ 430,000		$ 150,000
Average outstanding balance during the period	$ 99,946	$ 87,009	$ 78,960	$ 78,240
Average interest rate during the period (as percent)	0.44%	0.17%	0.42%	0.16%
Maximum outstanding at any month end during the period	$ 430,000	$ 182,000	$ 495,000	$ 387,000
Federal Home Loan Bank overnight advances | Weighted Average
FHLB advances
Interest rate (as percent)	0.40%		0.40%		0.35%