Accumulated Other Comprehensive Loss, Net of Tax (Schedule of Reclassifications from AOCI) (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Impairment loss on securities recognized in earnings										$ (149)	$ (110)	$ (1,145)
Income before income tax expense		$ 81,505	$ 76,262	$ 75,202	$ 70,481	$ 75,439	$ 76,365	$ 72,469	$ 73,488	303,450	297,761	291,699
Income tax expense		$ (23,845)	$ (24,445)	$ (24,599)	$ (23,434)	$ (23,627)	$ (24,995)	$ (20,426)	$ (23,984)	(96,323)	(93,032)	(91,973)
Earnings applicable to common shareholders										198,423	195,361	188,496
Net of tax										(9,421)	(9,303)	(4,312)
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										414	609	5,499
Impairment loss on securities recognized in earnings										(149)	(110)	(1,145)
Income before income tax expense										265	499	4,354
Available For Sale and Transferred Securities [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Net of tax										168	316	2,773
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										(97)	(183)	(1,581)
Earnings applicable to common shareholders										168	316	2,773
Derivative Instruments [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Net of tax										(5,087)	(5,686)	(5,182)
Derivative Instruments [Member] | Reclassification out of accumualted comprehensive income [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense										2,933	3,279	2,918
Earnings applicable to common shareholders										(5,087)	(5,686)	(5,182)
Total interest expense										(8,020)	(8,965)	(8,100)
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	[1]									(7,126)	(6,161)	(2,921)
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	[1]									(14)	(73)	(73)
Defined benefit pension and postretirement benefit plans [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total before tax										(7,140)	(6,234)	(2,994)
Tax benefit										2,638	2,301	1,091
Net of tax										$ (4,502)	$ (3,933)	$ (1,903)

[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).