Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Consolidated Statements of Comprehensive Income
Net income	$ 101,282	$ 99,473	$ 75,437
Unrealized gains (losses) on securities:
Unrealized holding gains (losses) arising during period	(6,821)	(5,142)	17,843
Tax effect	2,600	1,961	(6,804)
Reclassification adjustment for (gains) losses included in net income	(122)	489	2
Tax effect	47	(187)	(1)
Net of tax amount	(4,296)	(2,879)	11,040
Unrealized losses on derivative financial instruments qualifying as cash flow hedges:
Unrealized holding losses arising during period	(55)	(168)	(252)
Tax effect	21	64	96
Reclassification adjustment for losses included in interest expense	275	305	310
Tax effect	(105)	(116)	(118)
Net of tax amount	136	85	36
Change in pension plan obligation:
Change in pension and retiree medical plan obligation during period	(1,202)	(2,229)	(3,727)
Tax effect	453	897	1,526
Reclassification adjustment for changes included in net income	920	1,021	761
Tax effect	(351)	(389)	(290)
Net of tax amount	(180)	(700)	(1,730)
Other comprehensive income (loss), net of tax	(4,340)	(3,494)	9,346
Comprehensive income	$ 96,942	$ 95,979	$ 84,783