Accumulated Other Comprehensive Income (Loss) - Changes in Balances of Each Component (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss)
Beginning balance			$ 901,657
Other comprehensive income before reclassifications			7,878	$ 6,315
Amounts reclassified from accumulated other comprehensive income	$ (118)	$ (339)	(655)	(1,029)
Period change	(7,742)	(3,111)	7,223	5,286
Ending balance	1,283,120		1,283,120
Total
Accumulated Other Comprehensive Income (Loss)
Beginning balance			(13,581)	(1,362)
Period change			7,223
Ending balance	(6,358)	3,924	(6,358)	3,924
Unrealized Gains (Losses) on Securities Available for Sale
Accumulated Other Comprehensive Income (Loss)
Beginning balance			1,035	12,325
Other comprehensive income before reclassifications			8,124	7,863
Amounts reclassified from accumulated other comprehensive income	(216)	(545)	(973)	(1,652)
Period change			7,151	6,211
Ending balance	8,186	18,536	8,186	18,536
Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss)
Beginning balance			(1,774)	(2,347)
Other comprehensive income before reclassifications			(246)	(1,548)
Amounts reclassified from accumulated other comprehensive income	156	206	493	623
Period change			247	(925)
Ending balance	(1,527)	(3,272)	(1,527)	(3,272)
Unrealized Gains (Losses) on Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss)
Beginning balance			(12,842)	(11,340)
Other comprehensive income before reclassifications
Amounts reclassified from accumulated other comprehensive income			(175)
Period change			(175)	0
Ending balance	$ (13,017)	$ (11,340)	$ (13,017)	$ (11,340)