Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Consolidated Statements of Comprehensive Income
Net income	$ 87,554	$ 101,282	$ 99,473
Unrealized gains (losses) on securities:
Unrealized holding gains (losses) arising during period	(3,486)	(6,821)	(5,142)
Tax effect	1,329	2,600	1,961
Reclassification adjustment for (gains) losses and OTTI included in net income	(668)	(122)	489
Tax effect	255	47	(187)
Net of tax amount	(2,570)	(4,296)	(2,879)
Unrealized losses on derivative financial instruments qualifying as cash flow hedges:
Unrealized holding losses arising during period	(22)	(55)	(168)
Tax effect	9	21	64
Reclassification adjustment for losses included in interest expense	275	275	305
Tax effect	(105)	(105)	(116)
Net of tax amount	157	136	85
Change in pension plan obligation:
Change in pension and retiree medical plan obligation during period	(589)	(1,202)	(2,229)
Tax effect	224	453	897
Reclassification adjustment for changes included in net income	908	920	1,021
Tax effect	(346)	(351)	(389)
Net of tax amount	197	(180)	(700)
Other comprehensive income (loss), net of tax	(2,216)	(4,340)	(3,494)
Comprehensive income	$ 85,338	$ 96,942	$ 95,979