Allowance for loan losses - Troubled debt restructuring according to their accruing status (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Troubled Debt Restructuring
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	$ 95,846		$ 130,537
Commercial
Troubled Debt Restructuring
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	30,130		52,250
Construction
Troubled Debt Restructuring
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	57		5,530
Mortgages
Troubled Debt Restructuring
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	42,868		46,205
Leasing
Troubled Debt Restructuring
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	71		297
Consumer
Troubled Debt Restructuring
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	22,720		$ 26,255
Non Covered Loans
Troubled Debt Restructuring
Troubled debt restructurings	1,582,368	$ 1,515,889
Non Covered Loans | TDR
Troubled Debt Restructuring
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	80,388	112,999
Non Covered Loans | Commercial
Troubled Debt Restructuring
Troubled debt restructurings	350,722	360,679
Non Covered Loans | Commercial | TDR
Troubled Debt Restructuring
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	15,971	46,889
Non Covered Loans | Construction
Troubled Debt Restructuring
Troubled debt restructurings	274	1,788
Non Covered Loans | Construction | TDR
Troubled Debt Restructuring
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	57	56
Non Covered Loans | Mortgages
Troubled Debt Restructuring
Troubled debt restructurings	1,128,755	1,042,470
Non Covered Loans | Mortgages | TDR
Troubled Debt Restructuring
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	42,029	41,211
Non Covered Loans | Leasing
Troubled Debt Restructuring
Troubled debt restructurings	588	1,108
Non Covered Loans | Leasing | TDR
Troubled Debt Restructuring
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	71	320
Non Covered Loans | Consumer
Troubled Debt Restructuring
Troubled debt restructurings	102,029	109,844
Non Covered Loans | Consumer | TDR
Troubled Debt Restructuring
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	22,260	24,523
Non-Accruing | Non Covered Loans
Troubled Debt Restructuring
Troubled debt restructurings	265,696	284,558
Non-Accruing | Non Covered Loans | Commercial
Troubled Debt Restructuring
Troubled debt restructurings	116,508	130,921
Non-Accruing | Non Covered Loans | Construction
Troubled Debt Restructuring
Troubled debt restructurings	274	1,788
Non-Accruing | Non Covered Loans | Mortgages
Troubled Debt Restructuring
Troubled debt restructurings	132,755	135,758
Non-Accruing | Non Covered Loans | Leasing
Troubled Debt Restructuring
Troubled debt restructurings	245	440
Non-Accruing | Non Covered Loans | Consumer
Troubled Debt Restructuring
Troubled debt restructurings	15,914	15,651
Accruing | Non Covered Loans
Troubled Debt Restructuring
Troubled debt restructurings	1,316,672	1,231,331
Accruing | Non Covered Loans | Commercial
Troubled Debt Restructuring
Troubled debt restructurings	234,214	229,758
Accruing | Non Covered Loans | Construction
Troubled Debt Restructuring
Troubled debt restructurings	0	0
Accruing | Non Covered Loans | Mortgages
Troubled Debt Restructuring
Troubled debt restructurings	996,000	906,712
Accruing | Non Covered Loans | Leasing
Troubled Debt Restructuring
Troubled debt restructurings	343	668
Accruing | Non Covered Loans | Consumer
Troubled Debt Restructuring
Troubled debt restructurings	$ 86,115	$ 94,193