Accumulated Other Comprehensive Income (Loss) (Components of AOCI) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		$ (244,638)	$ (102,843)	$ (122,294)	$ (135,192)
Cumulative effect adjustments				(50,052)
Other comprehensive loss before reclassifications		(10,635)	9,895	(91,755)	41,218
Amounts reclassified from accumulated other comprehensive (loss) income		7,698	624	16,526	1,650
Net current period other comprehensive (loss) income		(2,937)	10,519	(75,229)	42,868
Ending balance		(247,575)	(92,324)	(247,575)	(92,324)
Unrealized Losses on Investment Securities Available for Sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	[1]	(57,496)	(16,648)	(30,945)	(45,875)
Cumulative effect adjustments	[1]			(29,752)
Other comprehensive loss before reclassifications	[1]	(10,635)	9,895	(7,434)	41,218
Amounts reclassified from accumulated other comprehensive (loss) income	[1]	0	(842)	0	(2,938)
Net current period other comprehensive (loss) income	[1]	(10,635)	9,053	(7,434)	38,280
Ending balance	[1]	(68,131)	(7,595)	(68,131)	(7,595)
Unrealized Losses On Securities Available For Sale Transferred To Held To Maturity
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	[1]	(80,876)	0	0	0
Cumulative effect adjustments	[1]			0
Other comprehensive loss before reclassifications	[1]	0	0	(84,321)	0
Amounts reclassified from accumulated other comprehensive (loss) income	[1]	5,007	0	8,452	0
Net current period other comprehensive (loss) income	[1]	5,007	0	(75,869)	0
Ending balance	[1]	(75,869)	0	(75,869)	0
Defined Benefit Pension Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	[1]	(106,266)	(86,195)	(91,349)	(89,317)
Cumulative effect adjustments	[1]			(20,300)
Other comprehensive loss before reclassifications	[1]	0	0	0	0
Amounts reclassified from accumulated other comprehensive (loss) income	[1]	2,691	1,466	8,074	4,588
Net current period other comprehensive (loss) income	[1]	2,691	1,466	8,074	4,588
Ending balance	[1]	$ (103,575)	$ (84,729)	$ (103,575)	$ (84,729)

[1] All amounts are net of tax.