MORTGAGE SERVICING RIGHTS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Data and assumptions used in fair value calculation [Abstract]
Unpaid principal balance	$ 6,384,649	$ 6,011,236	$ 5,686,756
Weighted-average prepayment speed (CPR) (in hundredths)	9.40%	10.30%	11.60%
Discount rate (annual percentage) (in hundredths)	9.80%	9.80%	9.80%
Weighted-average coupon interest rate (percentage) (in hundredths)	3.90%	4.00%	4.10%
Weighted-average remaining maturity (months)	323 months	319 months	314 months
Weighted-average servicing fee (basis points) (in hundredths)	0.267%	0.266%	0.265%
Contractually specified servicing fees, late fees, and ancillary fees [Abstract]
Contractual servicing fees	$ 16,900	$ 16,100	$ 15,200
Late and other ancillary fees	1,800	1,300	1,200
Closed End Loans for One-to-Four Family Residences, Secured by First Liens [Member]
Activity in mortgage servicing asset [Roll Forward]
Fair value at beginning of period	57,268	51,296
Additions [Abstract]
Origination of servicing assets	14,706	14,139
Changes in fair value [Abstract]
Due to payoffs/paydowns	(8,231)	(6,999)
Due to change in valuation inputs or assumptions used in the valuation model	1,526	(1,161)
Other changes in fair value	(6)	(7)
Fair value at end of period	$ 65,263	$ 57,268	$ 51,296