Accumulated Other Comprehensive Loss, Net of Tax (Schedule of Accumulated Other Comprehensive Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense	$ (26,451)	$ (20,743)	$ (52,592)	$ (40,818)
Earnings applicable to common shareholders	96,193	79,489	193,483	157,573
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Net of tax	0	0	0	0
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Net of tax	(1,056)	(1,386)	(2,091)	(2,779)
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] | Reclassification out of accumualted comprehensive income [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Other Nonoperating Expense	(1,408)	(1,861)	(2,799)	(3,732)
Interest Income, Other	(12)	0	(12)	0
Income tax expense	364	475	720	953
Earnings applicable to common shareholders	(1,056)	(1,386)	(2,091)	(2,779)
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	(1,429)	(1,493)	(2,859)	(2,778)
Defined benefit pension and postretirement benefit plans [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense	373	384	747	715
Net of tax	$ (1,056)	$ (1,109)	$ (2,112)	$ (2,063)