Accumulated Other Comprehensive Income (Loss) - Changes in Balances of Each Component (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss)
Beginning balance	$ 1,809,095	$ 1,501,636	$ 1,786,437	$ 1,408,260
Other comprehensive income before reclassifications			44,583	49,542
Amounts reclassified from accumulated other comprehensive income	(1,203)	(228)	(6,989)	(2,472)
Total other comprehensive income (loss), net of tax			37,594	47,070
Ending balance	1,833,656	1,749,012	1,833,656	1,749,012
Total
Accumulated Other Comprehensive Income (Loss)
Beginning balance	63,845	14,602	27,874	(21,422)
Ending balance	65,468	25,648	65,468	25,648
Unrealized Gains (Losses) on Securities Available for Sale
Accumulated Other Comprehensive Income (Loss)
Beginning balance			38,872	(6,343)
Other comprehensive income before reclassifications			45,729	50,545
Amounts reclassified from accumulated other comprehensive income			(7,503)	(2,667)
Total other comprehensive income (loss), net of tax			38,226	47,878
Ending balance	77,098	41,535	77,098	41,535
Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss)
Beginning balance			(1,141)	(559)
Other comprehensive income before reclassifications			(1,146)	(1,003)
Amounts reclassified from accumulated other comprehensive income			514	195
Total other comprehensive income (loss), net of tax			(632)	(808)
Ending balance	(1,773)	(1,367)	(1,773)	(1,367)
Unrealized Gains (Losses) on Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss)
Beginning balance			(9,857)	(14,520)
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
Ending balance	$ (9,857)	$ (14,520)	$ (9,857)	$ (14,520)