Accumulated Other Comprehensive Income (Loss) - Changes in Balances of Each Component (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss)
Beginning balance			$ 850,509
Other comprehensive income before reclassifications			6,315	$ (1,351)
Amounts reclassified from accumulated other comprehensive income	$ (339)	$ (490)	(1,029)	(633)
Period change	(3,111)	2,876	5,286	(1,984)
Ending balance	900,865		900,865
Total
Accumulated Other Comprehensive Income (Loss)
Beginning balance			(1,362)	(1,630)
Period change			5,286
Ending balance	3,924	(3,614)	3,924	(3,614)
Unrealized Gains (Losses) on Securities Available for Sale
Accumulated Other Comprehensive Income (Loss)
Beginning balance			12,325	14,098
Other comprehensive income before reclassifications			7,863	117
Amounts reclassified from accumulated other comprehensive income	(545)	(725)	(1,652)	(1,331)
Period change			6,211	(1,214)
Ending balance	18,536	12,884	18,536	12,884
Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss)
Beginning balance			(2,347)	(2,182)
Other comprehensive income before reclassifications			(1,548)	(1,468)
Amounts reclassified from accumulated other comprehensive income	206	235	623	698
Period change			(925)	(770)
Ending balance	(3,272)	(2,952)	(3,272)	(2,952)
Unrealized Gains (Losses) on Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss)
Beginning balance			(11,340)	(13,546)
Other comprehensive income before reclassifications
Amounts reclassified from accumulated other comprehensive income
Period change			0	0
Ending balance	$ (11,340)	$ (13,546)	$ (11,340)	$ (13,546)