Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 68,988	$ 52,014	$ 140,584	$ 99,618
Other comprehensive income, net of tax:
Unrealized holding gains (losses) on securities available-for-sale arising during the period	(11,622)	5,528	(39,229)	8,267
Reclassification adjustment for gains and losses on realized income	(3)	(77)	(3)	(167)
Tax effect	2,441	(1,908)	8,239	(2,835)
Net unrealized gains (losses) on securities available-for-sale, net of tax	(9,184)	3,543	(30,993)	5,265
Unrealized gains (losses) on interest rate swaps designated as cash flow hedges	4,102	(466)	12,065	(466)
Reclassification adjustment for losses included in net income	(589)	409	(347)	409
Tax effect	(738)	0	(2,461)	0
Net unrealized gains on interest rate swaps designated as cash flow hedges, net of tax	2,775	(57)	9,257	(57)
Adjustment for pension and other postretirement benefits	142	539	284	1,076
Tax effect	(30)	(189)	(60)	(377)
Net adjustment for pension and other postretirement benefits	112	350	224	699
Other comprehensive income (loss), net of tax	(6,297)	3,836	(21,512)	5,907
Total comprehensive income, net of tax	$ 62,691	$ 55,850	$ 119,072	$ 105,525