Accumulated Other Comprehensive Loss (Reclassifications out of AOCI) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Long-term obligations		$ 1,059,896	$ 944,675		$ 924,905
Securities gains (losses)		4,293	26,673		10,817
Income taxes		(219,946)	(125,585)		$ (122,028)
Amounts reclassified from accumulated other comprehensive income, net of tax	[1]	3,419	(12,351)
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]	4,293	26,673
Income taxes	[1]	(1,588)	(9,869)
Amounts reclassified from accumulated other comprehensive income, net of tax	[1],[2]	(2,705)	(16,804)
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]	(9,510)	(6,859)
Total before taxes		(9,720)	(7,069)	[1]
Income taxes	[1]	3,596	2,616
Amounts reclassified from accumulated other comprehensive income, net of tax	[1],[2]	$ 6,124	$ 4,453

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

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