Accumulated Other Comprehensive Loss (Reclassifications out of AOCI) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015		Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Long-term obligations		$ 944,675	$ 924,905		$ 710,097
Securities gains (losses)		26,673	10,817		29,096
Income taxes		(125,585)	(122,028)		$ (65,032)
Amounts reclassified from accumulated other comprehensive income, net of tax	[1]	(12,351)	(81)
Unrealized Loss on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive income, net of tax	[2]	0	0
Unrealized Gains on Investment Securities Available for Sale
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Securities gains (losses)	[1]	26,673	10,817
Income taxes	[1]	(9,869)	(4,138)
Amounts reclassified from accumulated other comprehensive income, net of tax	[1],[2]	(16,804)	(6,679)
Defined Benefit Pension Items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Employee Benefits, prior service costs	[1]	(210)	(210)
Reclassification adjustment for losses included in income before income taxes	[1]	(6,859)	(11,376)
Total before taxes		(7,069)	(11,586)	[1]
Income taxes	[1]	2,616	4,988
Amounts reclassified from accumulated other comprehensive income, net of tax	[1],[2]	$ 4,453	$ 6,598

[1] Amounts in parentheses indicate debits to profit/loss.

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