Accumulated Other Comprehensive Income (Loss) (Components of AOCI) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		$ (230,323)	$ (110,941)	$ (122,294)	$ (135,192)
Cumulative effect adjustments				(50,052)
Other comprehensive loss before reclassifications		(20,574)	8,646	(81,120)	31,323
Amounts reclassified from accumulated other comprehensive (loss) income		6,259	(548)	8,828	1,026
Net current period other comprehensive (loss) income		(14,315)	8,098	(72,292)	32,349
Ending balance		(244,638)	(102,843)	(244,638)	(102,843)
Unrealized Losses on Investment Securities Available for Sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	[1]	(121,243)	(23,183)	(30,945)	(45,875)
Cumulative effect adjustments	[1]			(29,752)
Other comprehensive loss before reclassifications	[1]	63,747	8,646	3,201	31,323
Amounts reclassified from accumulated other comprehensive (loss) income	[1]	0	(2,111)	0	(2,096)
Net current period other comprehensive (loss) income	[1]	63,747	6,535	3,201	29,227
Ending balance	[1]	(57,496)	(16,648)	(57,496)	(16,648)
Unrealized Losses On Securities Available For Sale Transferred To Held To Maturity
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	[1]	0	0	0	0
Cumulative effect adjustments	[1]			0
Other comprehensive loss before reclassifications	[1]	(84,321)	0	(84,321)	0
Amounts reclassified from accumulated other comprehensive (loss) income	[1]	3,445	0	3,445	0
Net current period other comprehensive (loss) income	[1]	(80,876)	0	(80,876)	0
Ending balance	[1]	(80,876)	0	(80,876)	0
Defined Benefit Pension Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	[1]	(109,080)	(87,758)	(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]	2,814	1,563	5,383	3,122
Net current period other comprehensive (loss) income	[1]	2,814	1,563	5,383	3,122
Ending balance	[1]	$ (106,266)	$ (86,195)	$ (106,266)	$ (86,195)

[1] All amounts are net of tax.