Other Comprehensive Income (Loss) (Reclassifications out of Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Interest expense	$ (38,850)	$ (32,474)
Income tax (expense) benefit	(30,209)	(33,847)
Investment securities gains, net	0	7,668
Salaries and other personnel expense	(113,720)	(107,191)
Net income available to common shareholders	100,607	69,298
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) | Net unrealized gains (losses) on cash flow hedges:
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Interest expense	0	(65)
Income tax (expense) benefit	0	25
Net income available to common shareholders	0	(40)
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) | Net unrealized gains (losses) on investment securities available for sale
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Income tax (expense) benefit	0	(2,952)
Investment securities gains, net	0	7,668
Net income available to common shareholders	0	4,716
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) | Post-retirement unfunded health benefit
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Income tax (expense) benefit	(13)	(8)
Salaries and other personnel expense	34	20
Net income available to common shareholders	$ 21	$ 12