Accumulated Other Comprehensive Income (Loss) - Summary of Changes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	$ 1,786,437	$ 1,408,260	$ 1,303,463
Other comprehensive income before reclassifications	55,577	52,436
Amounts reclassified from accumulated other comprehensive income	(8,615)	(3,140)
Period change	46,962	49,296	(17,888)
Ending balance	1,875,645	1,786,437	1,408,260
Total
Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	27,874	(21,422)	(2,908)
Period change	46,962	49,296	(17,888)
Ending balance	74,836	27,874	(21,422)
Unrealized Gains (Losses) on Securities Available for Sale
Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	38,872	(6,343)
Other comprehensive income before reclassifications	58,513	48,703
Amounts reclassified from accumulated other comprehensive income	(9,397)	(3,488)
Period change	49,116	45,215
Ending balance	87,988	38,872	(6,343)
Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(1,141)	(559)
Other comprehensive income before reclassifications	(1,169)	(846)
Amounts reclassified from accumulated other comprehensive income	716	264
Period change	(453)	(582)
Ending balance	(1,594)	(1,141)	(559)
Unrealized Gains (Losses) on Defined Benefit Plans
Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(9,857)	(14,520)
Other comprehensive income before reclassifications	(1,767)	4,579
Amounts reclassified from accumulated other comprehensive income	66	84
Period change	(1,701)	4,663
Ending balance	$ (11,558)	$ (9,857)	$ (14,520)