Note Other comprehensive income (loss) (Reclassification out of accumulated other comprehensive income (loss)) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Personnel costs	$ (119,636)	$ (121,224)	$ (364,058)	$ (365,023)
Net (loss) gain and valuation adjustments on investment securities	103	349	284	1,932
Other-than-temporary impairment losses on available-for-sale debt securities	0	0	(8,299)	(209)
Mortgage banking activities	5,239	15,272	27,349	42,050
Income tax (benefit) expense	19,966	(15,839)	(48,772)	(80,550)
Net Income	20,664	46,810	209,835	220,796
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	0	0	(8,299)	(209)
Net Income	(2,901)	(3,458)	(16,331)	(10,876)
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)	(4,538)	(13,969)	(13,611)
Income tax (benefit) expense	1,815	1,770	5,446	5,310
Net Income	(2,841)	(2,768)	(8,523)	(8,301)
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)	(5,488)	(16,819)	(16,461)
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	2,850	2,850
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	103	349	284	349
Total before tax	103	349	(8,015)	140
Income tax (benefit) expense	(21)	(33)	1,502	9
Net Income	82	316	(6,513)	149
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	(232)	(1,650)	(2,122)	(4,466)
Total before tax	(232)	(1,650)	(2,122)	(4,466)
Income tax (benefit) expense	90	644	827	1,742
Net Income	$ (142)	$ (1,006)	$ (1,295)	$ (2,724)