Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 70,397	$ 40,459	$ 210,981	$ 140,077
Other comprehensive income, net of tax:
Unrealized holding gains (losses) on securities available-for-sale arising during the period	(16,634)	1,376	(55,863)	9,643
Reclassification adjustment for gains on realized income	0	(1)	(3)	(168)
Tax effect	3,493	(481)	11,732	(3,316)
Net unrealized gains (losses) on securities available-for-sale, net of tax	(13,141)	894	(44,134)	6,159
Unrealized gains on interest rate swaps designated as cash flow hedges	4,111	550	16,176	84
Reclassification adjustment for (gains) losses included in net income	(662)	575	(1,009)	984
Tax effect	(724)	(374)	(3,185)	(374)
Net unrealized gains on interest rate swaps designated as cash flow hedges, net of tax	2,725	751	11,982	694
Plan remeasurement	0	11,238	0	11,238
Adjustment for pension and other postretirement benefits	142	537	426	1,613
Tax effect	(30)	(4,121)	(90)	(4,498)
Net adjustment for pension and other postretirement benefits	112	7,654	336	8,353
Other comprehensive income (loss), net of tax	(10,304)	9,299	(31,816)	15,206
Total comprehensive income, net of tax	$ 60,093	$ 49,758	$ 179,165	$ 155,283