Accumulated Other Comprehensive Income (Loss) - Summary of Changes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	$ 901,657	$ 850,509	$ 726,827
Other comprehensive income before reclassifications	12,340	(9,797)
Amounts reclassified from accumulated other comprehensive income	(1,667)	(2,422)	(1,006)
Period change	10,673	(12,219)	268
Ending balance	1,303,463	901,657	850,509
Total
Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(13,581)	(1,362)	(1,630)
Period change	10,673	(12,219)	268
Ending balance	(2,908)	(13,581)	(1,362)
Unrealized Gains (Losses) on Securities Available for Sale
Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	1,035	12,325
Other comprehensive income before reclassifications	9,645	(9,087)
Amounts reclassified from accumulated other comprehensive income	(1,710)	(2,203)	(1,736)
Period change	7,935	(11,290)
Ending balance	8,970	1,035	12,325
Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(1,774)	(2,347)
Other comprehensive income before reclassifications	9	(240)
Amounts reclassified from accumulated other comprehensive income	640	813	928
Period change	649	573
Ending balance	(1,125)	(1,774)	(2,347)
Unrealized Gains (Losses) on Defined Benefit Plans
Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(12,842)	(11,340)
Other comprehensive income before reclassifications	2,686	(470)
Amounts reclassified from accumulated other comprehensive income	(597)	(1,032)
Period change	2,089	(1,502)
Ending balance	$ (10,753)	$ (12,842)	$ (11,340)