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, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	$ 1,269,690	$ 1,268,200	$ 1,241,193	$ 1,231,868	$ 1,268,200	$ 1,231,868
Total Other Comprehensive Income (Loss)	16,454	7,165	(2,758)	(8,905)	23,619	(11,663)
Reclassification of the Income Tax Effect of the Tax Reform from AOCI to RE			0	0
Balance at End of Period	1,285,948	1,269,690	1,247,717	1,241,193	1,285,948	1,247,717
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(35,764)	(36,010)	(33,468)	(27,715)	(36,010)	(27,715)
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)	245		216		491	432
Total Other Comprehensive Income (Loss)	245		216		491	432
Reclassification of the Income Tax Effect of the Tax Reform from AOCI to RE						(5,969)
Balance at End of Period	(35,519)	(35,764)	(33,252)	(33,468)	(35,519)	(33,252)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at Beginning of Period	(43,878)	(51,043)	(51,097)	(34,715)	(51,043)	(34,715)
Other Comprehensive Income (Loss) Before Reclassifications	16,030		(3,399)		22,611	(13,004)
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)	424		641		1,008	1,341
Total Other Comprehensive Income (Loss)	16,454	7,165	(2,758)	(8,905)	23,619	(11,663)
Reclassification of the Income Tax Effect of the Tax Reform from AOCI to RE			0	(7,477)		(7,477)
Balance at End of Period	(27,424)	(43,878)	(53,855)	(51,097)	(27,424)	(53,855)
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	(3,819)	(10,447)	(11,932)	(1,915)	(10,447)	(1,915)
Other Comprehensive Income (Loss) Before Reclassifications	16,030		(3,399)		22,611	(13,004)
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)	(93)		0		(46)	0
Total Other Comprehensive Income (Loss)	12,678		(3,399)		19,306	(13,004)
Reclassification of the Income Tax Effect of the Tax Reform from AOCI to RE						(412)
Balance at End of Period	8,859	(3,819)	(15,331)	(11,932)	8,859	(15,331)
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	(4,295)	(4,586)	(5,697)	(5,085)	(4,586)	(5,085)
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)	272		425		563	909
Total Other Comprehensive Income (Loss)	3,531		425		3,822	909
Reclassification of the Income Tax Effect of the Tax Reform from AOCI to RE						(1,096)
Balance at End of Period	$ (764)	$ (4,295)	$ (5,272)	$ (5,697)	$ (764)	$ (5,272)