Accumulated Other Comprehensive Loss, Net of Tax (Schedule of Reclassifications from AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Impairment loss on securities recognized in earnings									$ (126)	$ (149)	$ (110)
Income before income tax expense	$ 86,922	$ 94,777	$ 90,669	$ 81,422	$ 81,505	$ 76,262	$ 75,202	$ 70,481	353,790	303,450	297,761
Income tax expense	(17,029)	$ (30,281)	$ (29,090)	$ (21,951)	$ (23,845)	$ (24,445)	$ (24,599)	$ (23,434)	(98,351)	(96,323)	(93,032)
Earnings applicable to common shareholders	$ 67,710								246,831	198,423	195,361
Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member] | Reclassification out of accumualted comprehensive income [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Gain on sale of investment securities, net									0	414	609
Impairment loss on securities recognized in earnings									0	(149)	(110)
Income before income tax expense									0	265	499
Available For Sale and Transferred Securities [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Net of tax									0	168	316
Available For Sale and Transferred Securities [Member] | Reclassification out of accumualted comprehensive income [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense									0	(97)	(183)
Earnings applicable to common shareholders									0	168	316
Derivative Instruments [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Net of tax									(4,384)	(5,087)	(5,686)
Derivative Instruments [Member] | Reclassification out of accumualted comprehensive income [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense									2,776	2,933	3,279
Earnings applicable to common shareholders									(4,384)	(5,087)	(5,686)
Total interest expense									(7,160)	(8,020)	(8,965)
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total before tax									(6,612)	(7,126)	(6,161)	[1]
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total before tax									0	(14)	(73)	[1]
Defined benefit pension and postretirement benefit plans [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total before tax									(6,612)	(7,140)	(6,234)
Tax benefit									2,575	2,638	2,301
Net of tax									$ (4,037)	$ (4,502)	$ (3,933)

[1] These accumulated other comprehensive income components are included in the computation of net periodic benefit cost (see Note 17 Retirement Benefit Plans for further details).