Accumulated Other Comprehensive Income (Loss) - Changes in Balances of Each Component (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income (Loss)
Beginning balance	$ 901,657
Other comprehensive income before reclassifications	10,133	$ 3,865
Amounts reclassified from accumulated other comprehensive income	(274)	(437)
Period change	9,859	3,428
Ending balance	929,470
Total
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(13,581)	(1,362)
Period change	9,859
Ending balance	(3,722)	2,066
Unrealized Gains (Losses) on Securities Available for Sale
Accumulated Other Comprehensive Income (Loss)
Beginning balance	1,035	12,325
Other comprehensive income before reclassifications	10,110	5,135
Amounts reclassified from accumulated other comprehensive income	(389)	(648)
Period change	9,721	4,487
Ending balance	10,756	16,812
Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(1,774)	(2,347)
Other comprehensive income before reclassifications	23	(1,270)
Amounts reclassified from accumulated other comprehensive income	174	211
Period change	197	(1,059)
Ending balance	(1,577)	(3,406)
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	(59)
Period change	(59)	0
Ending balance	$ (12,901)	$ (11,340)