INCOME TAXES (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 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	$ 9,733	$ (302)	$ 21,061	$ 20,866
Excess tax benefit on share-based compensation	(2,273)	(477)	(4,340)	(3,711)
Tax-exempt interest income	(375)	(1,187)	(1,460)	(3,525)
Dividends received deduction	(150)	(553)	(516)	(1,488)
ESOP dividends paid deduction	(139)	(240)	(423)	(724)
Other items, net	181	163	1,128	429
Income tax expense (benefit)	$ 6,977	$ (2,596)	$ 15,450	$ 11,847
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	(4.90%)	55.30%	(4.30%)	(6.20%)
Effective rate reduction due to tax exempt interest income (as a percent)	(0.80%)	137.70%	(1.50%)	(5.90%)
Effective rate reduction due to dividend received (as a percent)	(0.30%)	64.20%	(0.50%)	(2.50%)
Effective rate reduction due to dividend paid to ESOP (as a percent)	(0.30%)	27.90%	(0.40%)	(1.20%)
Effective rate reduction due to other items, net (as a percent)	0.40%	(18.90%)	1.10%	0.70%
Effective Income Tax Rate Reconciliation, Percent, Total	15.10%	301.20%	15.40%	19.90%