INCOME TAXES (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reconciliation of income tax expense reported to amount computed by applying the U.S. federal tax rate
Provision for income taxes at the statutory rate of 21% in 2018 and 35% in 2017	$ 8,308	$ 11,912	$ 11,327	$ 21,168
Excess tax benefit on share-based compensation	(924)	(2,791)	(2,067)	(3,234)
Tax-exempt interest income	(504)	(1,160)	(1,085)	(2,338)
Dividends received deduction	(169)	(405)	(366)	(935)
ESOP dividends paid deduction	(145)	(251)	(284)	(484)
Other items, net	(255)	523	948	266
Total tax expense	$ 6,311	$ 7,828	$ 8,473	$ 14,443
Reconciliation of income tax expense rate to the U.S. federal tax rate
U.S. federal tax rate (as a percent)	21.00%	35.00%	21.00%	35.00%
Excess tax benefit on share-based compensation	(2.30%)	(8.20%)	(3.90%)	(5.30%)
Effective rate reduction due to tax exempt interest income (as a percent)	(1.30%)	(3.40%)	(2.00%)	(3.90%)
Effective rate reduction due to dividend received (as a percent)	(0.40%)	(1.20%)	(0.70%)	(1.50%)
Effective rate reduction due to dividend paid to ESOP (as a percent)	(0.40%)	(0.70%)	(0.50%)	(0.80%)
Effective rate reduction due to other items, net (as a percent)	(0.60%)	1.50%	1.80%	0.40%
Effective Income Tax Rate Reconciliation, Percent, Total	16.00%	23.00%	15.70%	23.90%