FEDERAL HOME LOAN BANK ADVANCES - SHORT-TERM FHLB ADVANCES (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
FHLB advances
Federal Home Loan Bank advances	$ 1,002,500		$ 1,002,500		$ 802,500
Interest rate (as percent)	1.36%		1.36%
Federal Home Loan Bank overnight advances
FHLB advances
Federal Home Loan Bank advances	$ 625,000		$ 625,000		$ 285,000
Average outstanding balance during the period	$ 114,121	$ 128,846	$ 111,740	$ 68,352
Average interest rate during the year (as percent)	1.06%	0.40%	0.88%	0.40%
Maximum outstanding at any month end during the period	$ 625,000	$ 495,000	$ 625,000	$ 495,000
Federal Home Loan Bank overnight advances | Weighted Average
FHLB advances
Interest rate (as percent)	1.17%		1.17%		0.64%