Accumulated Other Comprehensive Income (Loss) (Components of AOCI) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jan. 01, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		$ (206,860)	$ (230,323)	$ (235,187)	$ (122,294)
Cumulative effect of new accounting principle in period of adoption						$ (50,051)
Net unrealized gains arising during period		23,848	(20,574)	45,463	(81,121)
Amounts reclassified from accumulated other comprehensive loss		2,281	6,259	8,993	8,828
Net current period other comprehensive income		26,129	(14,315)	54,456	(72,293)
Ending balance		(180,731)	(244,638)	(180,731)	(244,638)
Unrealized Losses on Investment Securities Available for Sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	[1]	(16,890)	(121,243)	(38,505)	(30,945)
Cumulative effect of new accounting principle in period of adoption	[1]					(29,751)
Net unrealized gains arising during period	[1]	23,848	63,747	45,463	3,200
Amounts reclassified from accumulated other comprehensive loss	[1]	(4,404)	0	(4,404)	0
Net current period other comprehensive income	[1]	19,444	63,747	41,059	3,200
Ending balance	[1]	2,554	(57,496)	2,554	(57,496)
Unrealized Losses On Securities Available For Sale Transferred To Held To Maturity
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	[1]	(66,558)	0	(71,149)	0
Cumulative effect of new accounting principle in period of adoption	[1]					0
Net unrealized gains arising during period	[1]	0	(84,321)	0	(84,321)
Amounts reclassified from accumulated other comprehensive loss	[1]	4,579	3,445	9,170	3,445
Net current period other comprehensive income	[1]	4,579	(80,876)	9,170	(80,876)
Ending balance	[1]	(61,979)	(80,876)	(61,979)	(80,876)
Defined Benefit Pension Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	[1]	(123,412)	(109,080)	(125,533)	(91,349)
Cumulative effect of new accounting principle in period of adoption	[1]					$ (20,300)
Net unrealized gains arising during period	[1]	0	0	0	0
Amounts reclassified from accumulated other comprehensive loss	[1]	2,106	2,814	4,227	5,383
Net current period other comprehensive income	[1]	2,106	2,814	4,227	5,383
Ending balance	[1]	$ (121,306)	$ (106,266)	$ (121,306)	$ (106,266)

[1] All amounts are net of tax. Amounts in parentheses indicate debits.