Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 52,014	$ 25,775	$ 99,618	$ 49,380
Other comprehensive income, net of tax:
Unrealized holding gains on securities available-for-sale arising during the period	5,528	2,121	8,267	6,320
Reclassification adjustment for gains on realized income	(77)	(18)	(167)	(37)
Tax effect	(1,908)	(736)	(2,835)	(2,199)
Net unrealized gains on securities available-for-sale, net of tax	3,543	1,367	5,265	4,084
Unrealized losses on interest rate swaps designated as cash flow hedges	(466)	0	(466)	0
Reclassification adjustment for losses included in net income	409	0	409	0
Net unrealized losses on interest rate swaps designated as cash flow hedges, net of tax	(57)	0	(57)	0
Adjustment for pension and other postretirement benefits	539	(577)	1,076	(1,154)
Tax effect	(189)	202	(377)	404
Net adjustment for pension and other postretirement benefits	350	(375)	699	(750)
Other comprehensive income, net of tax	3,836	992	5,907	3,334
Total comprehensive income, net of tax	$ 55,850	$ 26,767	$ 105,525	$ 52,714