Accumulated Other Comprehensive Income (Loss) (Components of AOCI) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption				$ (50,051)
Beginning balance		$ (235,187)	$ (122,294)
Net unrealized gains arising during period		21,615	(60,547)
Amounts reclassified from accumulated other comprehensive loss		6,712	2,569
Net current period other comprehensive income		28,327	(57,978)
Ending balance		(206,860)	(230,323)
Unrealized Losses on Investment Securities Available for Sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption				(29,751)
Beginning balance	[1]	(38,505)	(30,945)
Net unrealized gains arising during period	[1]	21,615	(60,547)
Amounts reclassified from accumulated other comprehensive loss	[1]	0	0
Net current period other comprehensive income	[1]	21,615	(60,547)
Ending balance	[1]	(16,890)	(121,243)
Unrealized Losses On Securities Available For Sale Transferred To Held To Maturity
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption				0
Beginning balance	[1]	(71,149)	0
Net unrealized gains arising during period	[1]	0	0
Amounts reclassified from accumulated other comprehensive loss	[1]	4,591	0
Net current period other comprehensive income	[1]	4,591	0
Ending balance	[1]	(66,558)	0
Defined Benefit Pension Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption				$ (20,300)
Beginning balance	[1]	(125,533)	(91,349)
Net unrealized gains arising during period	[1]	0	0
Amounts reclassified from accumulated other comprehensive loss	[1]	2,121	2,569
Net current period other comprehensive income	[1]	2,121	2,569
Ending balance	[1]	$ (123,412)	$ (109,080)

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