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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Personnel costs	$ (124,332)	$ (116,948)	$ (250,184)	$ (240,688)
Other-than-temporary impairment losses on available-for-sale debt securities	0	(8,299)	0	(8,299)
Mortgage banking activities	10,071	10,741	22,139	22,110
Income tax (benefit) expense	28,560	(35,732)	6,405	(68,738)
Net Income	279,783	96,226	371,107	189,171
Reclassification out of accumulated other comprehensive income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net Income	(2,910)	(10,197)	(4,893)	(13,430)
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	(4,517)	(4,656)	(9,036)	(9,313)
Income tax (benefit) expense	1,760	1,815	3,523	3,631
Net Income	(2,757)	(2,841)	(5,513)	(5,682)
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	(5,385)	(5,606)	(10,771)	(11,213)
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	868	950	1,735	1,900
Reclassification out of accumulated other comprehensive income | Unrealized holding gains (losses) on debt securities
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Total before tax	0	(8,299)	0	(8,299)
Income tax (benefit) expense	0	1,559	0	1,559
Net Income	0	(6,740)	0	(6,740)
Reclassification out of accumulated other comprehensive income | Unrealized holding gains (losses) on debt securities | Other-than-temporary impairment
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Other-than-temporary impairment losses on available-for-sale debt securities	0	(8,299)	0	(8,299)
Reclassification out of accumulated other comprehensive income | Unrealized holding gains (losses) on equity securities
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Total before tax	0	19	0	181
Income tax (benefit) expense	0	(4)	0	(36)
Net Income	0	15	0	145
Reclassification out of accumulated other comprehensive income | Unrealized holding gains (losses) on equity securities | Realized gain on sale of equity securities
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net gain on equity securities	0	19	0	181
Reclassification out of accumulated other comprehensive income | Unrealized net (losses) gains on cash flow hedges
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Total before tax	(250)	(1,035)	1,017	(1,890)
Income tax (benefit) expense	97	404	(397)	737
Net Income	(153)	(631)	620	(1,153)
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	$ (250)	$ (1,035)	$ 1,017	$ (1,890)