Accumulated Other Comprehensive Income (Loss) (Components of AOCI) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		$ (122,294)	$ (135,192)
Other comprehensive income before reclassifications		(60,546)	22,677
Amounts reclassified from accumulated other comprehensive (loss) income		2,569	1,574
Net current period other comprehensive income		(57,977)	24,251
Ending balance		(230,323)	(110,941)
Unrealized Gains on Investment Securities Available for Sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	[1]	(30,945)	(45,875)
Other comprehensive income before reclassifications	[1]	(60,546)	22,677
Amounts reclassified from accumulated other comprehensive (loss) income	[1]	0	15
Net current period other comprehensive income	[1]	(60,546)	22,692
Ending balance	[1]	(121,243)	(23,183)
Defined Benefit Pension Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	[1]	(91,349)	(89,317)
Other comprehensive income before reclassifications	[1]	0	0
Amounts reclassified from accumulated other comprehensive (loss) income	[1]	2,569	1,559
Net current period other comprehensive income	[1]	2,569	1,559
Ending balance	[1]	$ (109,080)	$ (87,758)

[1] All amounts are net of tax.