Note Allowance for loan losses (Troubled debt restructuring according to their accruing status) (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	$ 117,015		$ 118,072
Commercial
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	45,028		51,276
Construction
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	474		0
Mortgages
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	46,915		43,264
Leasing
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	448		522
Consumer
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	24,150		$ 23,010
Non Covered Loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	1,343,490	$ 1,258,327
Non Covered Loans | TDR
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	112,280	104,581
Non Covered Loans | Commercial
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	285,205	220,846
Non Covered Loans | Commercial | TDR
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	40,293	32,472
Non Covered Loans | Construction
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	4,293	0
Non Covered Loans | Construction | TDR
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	474	0
Non Covered Loans | Mortgages
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	944,987	930,076
Non Covered Loans | Mortgages | TDR
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	46,915	48,832
Non Covered Loans | Leasing
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	1,361	1,256
Non Covered Loans | Leasing | TDR
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	448	475
Non Covered Loans | Consumer
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	107,644	106,149
Non Covered Loans | Consumer | TDR
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	24,150	22,802
Covered loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	6,130	5,885
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	6,130	5,885
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	210,580	199,050
Non-Accruing | Non Covered Loans | Commercial
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	57,951	59,626
Non-Accruing | Non Covered Loans | Construction
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	4,293	0
Non-Accruing | Non Covered Loans | Mortgages
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	133,626	126,798
Non-Accruing | Non Covered Loans | Leasing
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	346	393
Non-Accruing | Non Covered Loans | Consumer
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	14,364	12,233
Non-Accruing | Covered loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	2,768	3,227
Non-Accruing | Covered loans | Mortgages
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	2,768	3,227
Accruing | Non Covered Loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	1,132,910	1,059,277
Accruing | Non Covered Loans | Commercial
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	227,254	161,220
Accruing | Non Covered Loans | Construction
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	0	0
Accruing | Non Covered Loans | Mortgages
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	811,361	803,278
Accruing | Non Covered Loans | Leasing
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	1,015	863
Accruing | Non Covered Loans | Consumer
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	93,280	93,916
Accruing | Covered loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	3,362	2,658
Accruing | Covered loans | Mortgages
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	$ 3,362	$ 2,658