Accumulated Other Comprehensive Income (Loss) - Changes in Balances of Each Component (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Accumulated Other Comprehensive Income (Loss)
Beginning balance	$ 1,455,848	$ 1,313,073	$ 1,408,260	$ 1,303,463
Other comprehensive income before reclassifications			38,268	(19,394)
Amounts reclassified from accumulated other comprehensive income	(1,390)	(803)	(2,244)	(1,940)
Total other comprehensive income (loss), net of tax	16,197	(3,143)	36,024	(21,334)
Reclassification adjustment under ASU 2018-02				0
Ending balance	1,501,636	1,340,328	1,501,636	1,340,328
Total
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(1,595)	(21,725)	(21,422)	(2,908)
Total other comprehensive income (loss), net of tax	16,197	(3,143)	36,024	(21,334)
Reclassification adjustment under ASU 2018-02			(626)	(626)
Ending balance	14,602	(24,868)	14,602	(24,868)
Unrealized Gains (Losses) on Securities Available for Sale
Accumulated Other Comprehensive Income (Loss)
Beginning balance			(6,343)	8,970
Other comprehensive income before reclassifications			39,130	(20,268)
Amounts reclassified from accumulated other comprehensive income	(1,456)	(886)	(2,357)	(2,157)
Total other comprehensive income (loss), net of tax			36,773	(22,425)
Reclassification adjustment under ASU 2018-02			1,932
Ending balance	30,430	(11,523)	30,430	(11,523)
Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss)
Beginning balance			(559)
Other comprehensive income before reclassifications			(862)
Amounts reclassified from accumulated other comprehensive income	66		113
Total other comprehensive income (loss), net of tax			(749)
Ending balance	(1,308)		(1,308)
Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss)
Beginning balance				(1,125)
Other comprehensive income before reclassifications				874
Amounts reclassified from accumulated other comprehensive income		83		217
Total other comprehensive income (loss), net of tax				1,091
Reclassification adjustment under ASU 2018-02			(242)
Ending balance		(276)		(276)
Unrealized Gains (Losses) on Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss)
Beginning balance			(14,520)	(10,753)
Other comprehensive income before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive income			0	0
Total other comprehensive income (loss), net of tax			0	0
Reclassification adjustment under ASU 2018-02			(2,316)
Ending balance	$ (14,520)	$ (13,069)	$ (14,520)	$ (13,069)