Accumulated Other Comprehensive Loss, Net of Tax (Schedule of Accumulated Other Comprehensive Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense	$ (11,144)	$ (26,141)
Earnings applicable to common shareholders	36,021	97,549
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense	2	0
Net of tax	6	0
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member] | Reclassification out of accumualted comprehensive income [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Debt Securities, Available-for-sale, Gain (Loss)	8	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	(1,453)	(1,035)
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	(1,173)	(1,391)
Interest Income, Other	(794)	0
Income tax expense	514	356
Earnings applicable to common shareholders	(1,453)	(1,035)
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	(990)	(1,430)
Defined benefit pension and postretirement benefit plans [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense	261	374
Net of tax	$ (729)	$ (1,056)