Note Other comprehensive income (loss) (Reclassification out of accumulated other comprehensive income (loss)) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Personnel costs	$ (118,815)	$ (116,708)	$ (244,422)	$ (243,799)
Other operating income	18,151	12,866	37,279	28,411
Net (loss) gain and valuation adjustments on investment securities	19	1,583	181	1,583
Other-than-temporary impairment losses on available-for-sale debt securities	(8,299)	(209)	(8,299)	(209)
Mortgage banking activities	10,741	16,227	22,110	26,778
Income tax (benefit) expense	(35,732)	(32,446)	(68,738)	(64,711)
Net Income	96,226	88,987	189,171	173,986
Reclassification out of accumulated other comprehensive income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Other-than-temporary impairment losses on available-for-sale debt securities	(8,299)	(209)	(8,299)	(209)
Net Income	(10,197)	(3,709)	(13,430)	(7,418)
Reclassification out of accumulated other comprehensive income | Adjustment of pension and postretirement benefit plan
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Total before tax	(4,656)	(4,537)	(9,313)	(9,073)
Income tax (benefit) expense	1,815	1,770	3,631	3,540
Net Income	(2,841)	(2,767)	(5,682)	(5,533)
Reclassification out of accumulated other comprehensive income | Adjustment of pension and postretirement benefit plan | Amortization of net losses
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Personnel costs	(5,606)	(5,487)	(11,213)	(10,973)
Reclassification out of accumulated other comprehensive income | Adjustment of pension and postretirement benefit plan | Amortization of prior service credit
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Personnel costs	950	950	1,900	1,900
Reclassification out of accumulated other comprehensive income | Unrealized holding (losses) gain on investments | Realized gain (loss) on sale of securities
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net (loss) gain and valuation adjustments on investment securities	19	0	181	0
Total before tax	(8,280)	(209)	(8,118)	(209)
Income tax (benefit) expense	1,555	42	1,523	42
Net Income	(6,725)	(167)	(6,595)	(167)
Reclassification out of accumulated other comprehensive income | Unrealized net (losses) gains on cash flow hedges | Forward contracts
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Mortgage banking activities	(1,035)	(1,271)	(1,890)	(2,816)
Total before tax	(1,035)	(1,271)	(1,890)	(2,816)
Income tax (benefit) expense	(404)	(496)	(737)	(1,098)
Net Income	$ (631)	$ (775)	$ (1,153)	$ (1,718)