Accumulated Other Comprehensive Income (Loss) - Changes in Balances of Each Component (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss)
Beginning balance	$ 1,408,260	$ 1,303,463
Other comprehensive income before reclassifications	20,681	(17,054)
Amounts reclassified from accumulated other comprehensive income	(854)	(1,137)
Total other comprehensive income (loss), net of tax	19,827	(18,191)
Reclassification adjustment under ASU 2018-02		0
Ending balance	1,455,848	1,313,073
Total
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(21,422)	(2,908)
Total other comprehensive income (loss), net of tax	19,827	(18,191)
Reclassification adjustment under ASU 2018-02	(626)	(626)
Ending balance	(1,595)	(21,725)
Unrealized Gains (Losses) on Securities Available for Sale
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(6,343)	8,970
Other comprehensive income before reclassifications	20,990	(17,698)
Amounts reclassified from accumulated other comprehensive income	(901)	(1,271)
Total other comprehensive income (loss), net of tax	20,089	(18,969)
Reclassification adjustment under ASU 2018-02	1,932
Ending balance	13,746	(8,067)
Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(559)	(1,125)
Other comprehensive income before reclassifications	(309)	644
Amounts reclassified from accumulated other comprehensive income	47	134
Total other comprehensive income (loss), net of tax	(262)	778
Reclassification adjustment under ASU 2018-02	(242)
Ending balance	(821)	(589)
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)