Accumulated Other Comprehensive Income (Loss) - Changes in Balances of Each Component (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss)
Beginning balance	$ 1,303,463
Other comprehensive income before reclassifications	(17,054)	$ 10,133
Amounts reclassified from accumulated other comprehensive income	(1,137)	(274)
Total other comprehensive income (loss), net of tax	(18,191)	9,859
Reclassification adjustment under ASU 2018-02	(626)
Ending balance	1,313,073
Total
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(2,908)	(13,581)
Total other comprehensive income (loss), net of tax	(18,191)
Ending balance	(21,725)	(3,722)
Unrealized Gains (Losses) on Securities Available for Sale
Accumulated Other Comprehensive Income (Loss)
Beginning balance	8,970	1,035
Other comprehensive income before reclassifications	(17,698)	10,110
Amounts reclassified from accumulated other comprehensive income	(1,271)	(389)
Total other comprehensive income (loss), net of tax	(18,969)	9,721
Reclassification adjustment under ASU 2018-02	1,932
Ending balance	(8,067)	10,756
Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(1,125)	(1,774)
Other comprehensive income before reclassifications	644	23
Amounts reclassified from accumulated other comprehensive income	134	174
Total other comprehensive income (loss), net of tax	778	197
Reclassification adjustment under ASU 2018-02	(242)
Ending balance	(589)	(1,577)
Unrealized Gains (Losses) on Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(10,753)	(12,842)
Other comprehensive income before reclassifications
Amounts reclassified from accumulated other comprehensive income		(59)
Total other comprehensive income (loss), net of tax	0	(59)
Reclassification adjustment under ASU 2018-02	(2,316)
Ending balance	$ (13,069)	$ (12,901)