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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Impairment loss recognized in earnings	$ 0	$ 0	$ (126)	$ (149)
Income (loss) before income tax expense	94,777	76,262	266,868	221,945
Income tax expense	(30,281)	(24,445)	(81,322)	(72,478)
Earnings applicable to common shareholders	62,426	49,634	179,262	142,927
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	0	0	414
Impairment loss recognized in earnings	0	0	0	(149)
Income (loss) before income tax expense	0	0	0	265
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Net of tax	0	0	0	168
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] | Reclassification out of accumualted comprehensive income [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense	0	0	0	(97)
Earnings applicable to common shareholders	0	0	0	168
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Net of tax	(1,145)	(1,221)	(3,366)	(4,005)
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]
Income tax expense	665	704	1,950	2,309
Earnings applicable to common shareholders	(1,145)	(1,221)	(3,366)	(4,005)
Total interest expense	(1,810)	(1,925)	(5,316)	(6,314)
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	(1,587)	(1,780)	(4,959)	(5,343)
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	0	(4)	0	(11)
Defined benefit pension and postretirement benefit plans [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	(1,587)	(1,784)	(4,959)	(5,354)
Income tax expense	586	659	1,832	1,978
Net of tax	$ (1,001)	$ (1,125)	$ (3,127)	$ (3,376)