Accumulated Other Comprehensive Income (Loss) - Changes in Balances of Each Component (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss)
Beginning balance			$ 1,303,463
Other comprehensive income before reclassifications			(19,394)	$ 15,502
Amounts reclassified from accumulated other comprehensive income	$ (803)	$ (264)	(1,940)	(537)
Total other comprehensive income (loss), net of tax	(3,143)	5,106	(21,334)	14,965
Reclassification adjustment under ASU 2018-02			(626)
Ending balance	1,340,328		1,340,328
Total
Accumulated Other Comprehensive Income (Loss)
Beginning balance			(2,908)	(13,581)
Total other comprehensive income (loss), net of tax			(21,334)
Ending balance	(24,868)	1,384	(24,868)	1,384
Unrealized Gains (Losses) on Securities Available for Sale
Accumulated Other Comprehensive Income (Loss)
Beginning balance			8,970	1,035
Other comprehensive income before reclassifications			(20,268)	15,741
Amounts reclassified from accumulated other comprehensive income	(886)	(369)	(2,157)	(757)
Total other comprehensive income (loss), net of tax			(22,425)	14,984
Reclassification adjustment under ASU 2018-02			1,932
Ending balance	(11,523)	16,019	(11,523)	16,019
Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss)
Beginning balance			(1,125)	(1,774)
Other comprehensive income before reclassifications			874	(239)
Amounts reclassified from accumulated other comprehensive income	83	163	217	337
Total other comprehensive income (loss), net of tax			1,091	98
Reclassification adjustment under ASU 2018-02			(242)
Ending balance	(276)	(1,676)	(276)	(1,676)
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				(117)
Total other comprehensive income (loss), net of tax			0	(117)
Reclassification adjustment under ASU 2018-02			(2,316)
Ending balance	$ (13,069)	$ (12,959)	$ (13,069)	$ (12,959)