Accumulated Other Comprehensive Income (Loss) - Change in AOCI Components Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	$ 1,352,082	$ 1,327,929	$ 1,286,832	$ 1,285,948	$ 1,269,690	$ 1,268,200	$ 1,286,832	$ 1,268,200
Total Other Comprehensive Income (Loss)	(4,526)	8,104	41,933	5,650	16,454	7,165	45,511	29,269
Balance at End of Period	1,361,739	1,352,082	1,327,929	1,291,490	1,285,948	1,269,690	1,361,739	1,291,490
Unrealized Gains and Losses on Net Investment Securities | Available-for-sale Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	57,511		8,359	8,859		(10,447)	8,359	(10,447)
Other Comprehensive Income (Loss) Before Reclassifications	(4,976)			5,376			44,232	27,987
Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, before Tax								(3,259)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0			0			(56)	(46)
Total Other Comprehensive Income (Loss)	(4,976)			5,376			44,176	24,682
Balance at End of Period	52,535	57,511		14,235	8,859		52,535	14,235
Unrealized Gains and Losses on Net Investment Securities | Held-to-maturity Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(578)		(715)	(764)		(4,586)	(715)	(4,586)
Other Comprehensive Income (Loss) Before Reclassifications	0			0			0	0
Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, before Tax								3,259
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	76			29			213	592
Total Other Comprehensive Income (Loss)	76			29			213	3,851
Balance at End of Period	(502)	(578)		(735)	(764)		(502)	(735)
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(38,008)		(38,756)	(35,519)		(36,010)	(38,756)	(36,010)
Other Comprehensive Income (Loss) Before Reclassifications	0			0			0	0
Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, before Tax								0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	374			245			1,122	736
Total Other Comprehensive Income (Loss)	374			245			1,122	736
Balance at End of Period	(37,634)	(38,008)		(35,274)	(35,519)		(37,634)	(35,274)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	18,925	10,821	(31,112)	(27,424)	(43,878)	(51,043)	(31,112)	(51,043)
Other Comprehensive Income (Loss) Before Reclassifications	(4,976)			5,376			44,232	27,987
Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, before Tax								0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	450			274			1,279	1,282
Total Other Comprehensive Income (Loss)	(4,526)	8,104	41,933	5,650	16,454	7,165	45,511	29,269
Balance at End of Period	$ 14,399	$ 18,925	$ 10,821	$ (21,774)	$ (27,424)	$ (43,878)	$ 14,399	$ (21,774)