Note Allowance for loan losses (Troubled debt restructuring according to their accruing status) (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	$ 119,339		$ 115,310
Commercial
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	46,626		41,982
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	47,515		50,148
Leasing
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	362		487
Consumer
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	24,836		$ 22,693
Non Covered Loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	1,409,929	$ 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	117,143	104,581
Non Covered Loans | Commercial
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	330,039	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	45,277	32,472
Non Covered Loans | Construction
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	2,559	0
Non Covered Loans | Construction | TDR
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	0	0
Non Covered Loans | Mortgages
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	967,125	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	47,515	48,832
Non Covered Loans | Leasing
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	1,130	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	362	475
Non Covered Loans | Consumer
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	109,076	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	23,989	22,802
Covered loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	0	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	0	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	262,886	199,050
Non-Accruing | Non Covered Loans | Commercial
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	104,440	59,626
Non-Accruing | Non Covered Loans | Construction
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	2,559	0
Non-Accruing | Non Covered Loans | Mortgages
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	141,753	126,798
Non-Accruing | Non Covered Loans | Leasing
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	366	393
Non-Accruing | Non Covered Loans | Consumer
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	13,768	12,233
Non-Accruing | Covered loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	0	3,227
Non-Accruing | Covered loans | Mortgages
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	0	3,227
Accruing | Non Covered Loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	1,147,043	1,059,277
Accruing | Non Covered Loans | Commercial
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	225,599	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	825,372	803,278
Accruing | Non Covered Loans | Leasing
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	764	863
Accruing | Non Covered Loans | Consumer
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	95,308	93,916
Accruing | Covered loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	0	2,658
Accruing | Covered loans | Mortgages
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Troubled debt restructurings	$ 0	$ 2,658