Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 40,459	$ 11,484	$ 140,077	$ 60,864
Other comprehensive income, net of tax:
Unrealized holding gains (losses) on securities available-for-sale arising during the period	1,376	(1,238)	9,643	5,082
Reclassification adjustment for gains and losses on realized income	(1)	(16)	(168)	(53)
Tax effect	(481)	439	(3,316)	(1,760)
Net unrealized gains (losses) on securities available-for-sale, net of tax	894	(815)	6,159	3,269
Unrealized gains on interest rate swaps designated as cash flow hedges	550	0	84	0
Reclassification adjustment for losses included in net income	575	0	984	0
Tax effect	(374)	0	(374)	0
Net unrealized gains on interest rate swaps designated as cash flow hedges, net of tax	751	0	694	0
Plan remeasurement	11,238	(786)	11,238	(786)
Adjustment for pension and other postretirement benefits	537	2,895	1,613	1,742
Tax effect	(4,121)	(738)	(4,498)	(335)
Net adjustment for pension and other postretirement benefits	7,654	1,371	8,353	621
Other comprehensive income, net of tax	9,299	556	15,206	3,890
Total comprehensive income, net of tax	$ 49,758	$ 12,040	$ 155,283	$ 64,754