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	$ 27,259	$ 25,193	$ 72,253	$ 70,871
Other comprehensive income, before tax:
Change in net unrealized (losses) gains during period	(1,812)	(3,430)	15,553	15,241
Reclassification adjustment for net (gains) losses included in income	(771)	(225)	(778)	(312)
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity	465	490	1,394	1,394
Unrealized loss (gain) on derivatives	(658)	512	(1,095)	(2,490)
Reclassification adjustment for derivatives net (gains) losses included in income	1,128	0	1,128	0
Defined benefit post-retirement benefits plans:
Change in net actuarial (gain) loss	(176)	15	(1,090)	43
Other comprehensive income, before tax	(1,824)	(2,638)	15,112	13,876
Deferred tax expense (benefit) related to other comprehensive income	851	1,038	(5,961)	(5,460)
Other comprehensive (loss) income, net of tax	(973)	(1,600)	9,151	8,416
Comprehensive income, net of tax	$ 26,286	$ 23,593	$ 81,404	$ 79,287