Other Comprehensive Income/Loss - Schedule of Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Investment securities available-for sale:
Change in net unrealized gain during period, pre-tax	$ (6,300)	$ (1,800)	$ (33,900)	$ 15,600
Change in net unrealized gain during period, tax expense (benefit)	(1,700)	(800)	(8,900)	6,100
Change in net unrealized gain during period, net of tax	(4,600)	(1,000)	(25,000)	9,500
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity, pre-tax	500	500	1,500	1,400
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity, tax expense (benefit)	100	200	400	600
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity, net of tax	400	300	1,100	800
Defined benefits post-retirement benefit plan:
Change in net actuarial (gain) loss, pre-tax	(200)	(200)	(500)	(1,100)
Change in net actuarial (gain) loss, tax expense (benefit)	0	(100)	(100)	(400)
Change in net actuarial (gain) loss, net of tax amount	(200)	(100)	(400)	(700)
Other comprehensive income (loss), before tax	(6,000)	(1,800)	(32,900)	15,100
Total other comprehensive income, tax expense (benefit)	(1,600)	(800)	(8,600)	6,000
Total other comprehensive income, net of tax amount	(4,400)	(1,000)	(24,300)	9,100
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax	0	(600)	0	(1,100)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax	0	(300)	0	(400)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	0	(300)	0	(700)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax	0	(1,100)	0	(1,100)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax	0	(500)	0	(400)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0	(600)	0	(700)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax	0	800	0	800
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	0	300	0	300
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	$ 0	$ 500	$ 0	$ 500