Other Comprehensive Income (Loss) (Reclassifications out of Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Interest expense									$ (118,644)	$ (109,408)	$ (118,822)
Loss on early extinguishment of debt									(1,533)	0	0
Income tax (expense) benefit	$ (32,342)	$ (36,058)	$ (32,242)	$ (31,849)	$ (25,756)	$ (25,868)	$ (27,078)	$ (28,608)	(132,491)	(107,310)	(93,245)
Investment securities gains, net									2,769	1,331	2,945
Salaries and other personnel expense									(717,655)	(744,998)	(741,537)
Net income available to common shareholders	$ 55,839	$ 55,369	$ 53,233	$ 51,404	$ 50,612	$ 44,229	$ 44,313	$ 45,857	215,844	185,011	118,553
Net Unrealized Gains (Losses) on Cash Flow Hedges [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Interest expense									(448)	(448)	(447)
Loss on early extinguishment of debt									(73)	0	0
Income tax (expense) benefit									201	173	173
Net income available to common shareholders									(320)	(275)	(274)
Net Unrealized Gains (Losses) on Investment Securities Available for Sale [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Income tax (expense) benefit									(1,066)	(513)	(1,134)
Investment securities gains, net									2,769	1,331	2,945
Net income available to common shareholders									1,703	818	1,811
Post-retirement Unfunded Health Benefit [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Income tax (expense) benefit									(104)	(56)	(65)
Salaries and other personnel expense									272	144	170
Net income available to common shareholders									$ 168	$ 88	$ 105