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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Impairment loss recognized in earnings	$ (126)	$ 0	$ (126)	$ (149)
Income (loss) before income tax expense	90,669	75,202	172,091	145,683
Income tax expense	(29,090)	(24,599)	(51,041)	(48,033)
Earnings applicable to common shareholders	59,485	48,398	116,826	93,282
Net of tax	(2,217)	(2,320)	(4,347)	(4,867)
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	94	0	414
Impairment loss recognized in earnings	0	0	0	(149)
Income (loss) before income tax expense	0	94	0	265
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Net of tax	0	59	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	(35)	0	(97)
Earnings applicable to common shareholders	0	59	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	(1,123)	(1,284)	(2,221)	(2,784)
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	648	740	1,285	1,605
Earnings applicable to common shareholders	(1,123)	(1,284)	(2,221)	(2,784)
Total interest expense	(1,771)	(2,024)	(3,506)	(4,389)
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	(1,734)	(1,734)	(3,372)	(3,563)
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	0	(3)	0	(7)
Defined benefit pension and postretirement benefit plans [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	(1,734)	(1,737)	(3,372)	(3,570)
Income tax expense	640	642	1,246	1,319
Net of tax	$ (1,094)	$ (1,095)	$ (2,126)	$ (2,251)