Accumulated Other Comprehensive Income (Loss) (Change in AOCI Components net of tax) (Details 2) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	$ 1,327,929	$ 1,286,832	$ 1,269,690	$ 1,268,200	$ 1,286,832	$ 1,268,200
Total Other Comprehensive Income (Loss)	8,104	41,933	16,454	7,165	50,037	23,619
Balance at End of Period	1,352,082	1,327,929	1,285,948	1,269,690	1,352,082	1,285,948
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(38,382)	(38,756)	(35,764)	(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			0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	374		245		748	491
Total Other Comprehensive Income (Loss)	374		245		748	491
Balance at End of Period	(38,008)	(38,382)	(35,519)	(35,764)	(38,008)	(35,519)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	10,821	(31,112)	(43,878)	(51,043)	(31,112)	(51,043)
Other Comprehensive Income (Loss) Before Reclassifications	7,641		16,030		49,208	22,611
Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, before Tax			0			0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	463		424		829	1,008
Total Other Comprehensive Income (Loss)	8,104	41,933	16,454	7,165	50,037	23,619
Balance at End of Period	18,925	10,821	(27,424)	(43,878)	18,925	(27,424)
Available-for-sale Securities | Unrealized Gains and Losses on Net Investment Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	49,870	8,359	(3,819)	(10,447)	8,359	(10,447)
Other Comprehensive Income (Loss) Before Reclassifications	7,641		16,030		49,208	22,611
Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, before Tax			(3,259)			(3,259)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	0		(93)		(56)	(46)
Total Other Comprehensive Income (Loss)	7,641		12,678		49,152	19,306
Balance at End of Period	57,511	49,870	8,859	(3,819)	57,511	8,859
Held-to-maturity Securities | Unrealized Gains and Losses on Net Investment Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(667)	(715)	(4,295)	(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			3,259
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)	89		272		137	563
Total Other Comprehensive Income (Loss)	89		3,531		137	3,822
Balance at End of Period	$ (578)	$ (667)	$ (764)	$ (4,295)	$ (578)	$ (764)