Accumulated Other Comprehensive Income (Loss) - Changes in Balances of Each Component (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Accumulated Other Comprehensive Income (Loss)
Beginning balance	$ 850,509
Amounts reclassified from accumulated other comprehensive income	(437)	$ (436)
Period change	3,428	(285)
Ending balance	867,263
Total
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(1,362)	(1,630)
Other comprehensive income before reclassifications	3,865	151
Amounts reclassified from accumulated other comprehensive income	(437)	(436)
Period change	3,428	(285)
Ending balance	2,066	(1,915)
Unrealized Gains (Losses) on Securities Available for Sale
Accumulated Other Comprehensive Income (Loss)
Beginning balance	12,325	14,098
Other comprehensive income before reclassifications	5,135	980
Amounts reclassified from accumulated other comprehensive income	(648)	(666)
Period change	4,487	314
Ending balance	16,812	14,412
Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(2,347)	(2,182)
Other comprehensive income before reclassifications	(1,270)	(829)
Amounts reclassified from accumulated other comprehensive income	211	230
Period change	(1,059)	(599)
Ending balance	(3,406)	(2,781)
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)