Note Allowance for loan losses (Troubled debt restructuring according to their accruing status) (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	$ 115,191		$ 129,057
Commercial
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	40,863		58,527
Construction
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0		0
Mortgages
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	51,421		45,557
Leasing
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	450		540
Consumer
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	22,457		$ 24,433
Non Covered Loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	1,263,875	$ 1,246,577
Non Covered Loans | TDR
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	111,799	109,812
Non Covered Loans | Commercial
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	229,841	260,044
Non Covered Loans | Commercial | TDR
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	37,471	40,810
Non Covered Loans | Mortgages
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	924,902	871,997
Non Covered Loans | Mortgages | TDR
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	51,421	44,610
Non Covered Loans | Leasing
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	1,333	1,817
Non Covered Loans | Leasing | TDR
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	450	535
Non Covered Loans | Consumer
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	107,799	112,719
Non Covered Loans | Consumer | TDR
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	22,457	23,857
Covered loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	5,937	5,530
Covered loans | TDR
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0	0
Covered loans | Mortgages
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	5,937	5,530
Covered loans | Mortgages | TDR
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0	0
Non-Accruing | Non Covered Loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	210,290	223,104
Non-Accruing | Non Covered Loans | Commercial
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	66,492	83,157
Non-Accruing | Non Covered Loans | Mortgages
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	131,424	127,071
Non-Accruing | Non Covered Loans | Leasing
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	370	434
Non-Accruing | Non Covered Loans | Consumer
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	12,004	12,442
Non-Accruing | Covered loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	2,617	2,580
Non-Accruing | Covered loans | Mortgages
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	2,617	2,580
Accruing | Non Covered Loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	1,053,585	1,023,473
Accruing | Non Covered Loans | Commercial
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	163,349	176,887
Accruing | Non Covered Loans | Mortgages
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	793,478	744,926
Accruing | Non Covered Loans | Leasing
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	963	1,383
Accruing | Non Covered Loans | Consumer
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	95,795	100,277
Accruing | Covered loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	3,320	2,950
Accruing | Covered loans | Mortgages
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	$ 3,320	$ 2,950