Accumulated Other Comprehensive Income (Loss) - Changes in Balances of Each Component (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss)
Beginning balance			$ 1,303,463
Other comprehensive income before reclassifications			(28,993)	$ 7,878
Amounts reclassified from accumulated other comprehensive income	$ (942)	$ (118)	(2,882)	(655)
Total other comprehensive income (loss), net of tax	(10,541)	(7,742)	(31,875)	7,223
Reclassification adjustment under ASU 2018-02			(626)
Ending balance	1,361,426		1,361,426
Total
Accumulated Other Comprehensive Income (Loss)
Beginning balance			(2,908)	(13,581)
Total other comprehensive income (loss), net of tax			(31,875)
Ending balance	(35,409)	(6,358)	(35,409)	(6,358)
Unrealized Gains (Losses) on Securities Available for Sale
Accumulated Other Comprehensive Income (Loss)
Beginning balance			8,970	1,035
Other comprehensive income before reclassifications			(30,032)	8,124
Amounts reclassified from accumulated other comprehensive income	(1,015)	(216)	(3,173)	(973)
Total other comprehensive income (loss), net of tax			(33,205)	7,151
Reclassification adjustment under ASU 2018-02			1,932
Ending balance	(22,303)	8,186	(22,303)	8,186
Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss)
Beginning balance			(1,125)	(1,774)
Other comprehensive income before reclassifications			1,039	(246)
Amounts reclassified from accumulated other comprehensive income	73	156	291	493
Total other comprehensive income (loss), net of tax			1,330	247
Reclassification adjustment under ASU 2018-02			(242)
Ending balance	(37)	(1,527)	(37)	(1,527)
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				(175)
Total other comprehensive income (loss), net of tax			0	(175)
Reclassification adjustment under ASU 2018-02			(2,316)
Ending balance	$ (13,069)	$ (13,017)	$ (13,069)	$ (13,017)

Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect

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Amount after tax, before reclassification adjustments of other comprehensive income (loss).

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