Accumulated Other Comprehensive Loss, Net of Tax (Schedule of Accumulated Other Comprehensive Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense	$ (25,411)	$ (13,700)	$ (78,003)	$ (54,518)
Earnings applicable to common shareholders	91,442	97,460	284,919	255,040
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	(1,387)	(1,438)	(3,478)	(4,217)
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	(1,375)	(1,932)	(4,174)	(5,664)
Interest Income, Other	(515)	0	(527)	0
Income tax expense	503	494	1,223	1,447
Earnings applicable to common shareholders	(1,387)	(1,438)	(3,478)	(4,217)
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	(1,421)	(3,431)	(4,280)	(6,209)
Defined benefit pension and postretirement benefit plans [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense	372	883	1,119	1,598
Net of tax	$ (1,049)	$ (2,548)	$ (3,161)	$ (4,611)