Portfolio loans - Summary of risk grades segregated by category (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Loans
Portfolio loans	$ 6,745,499	$ 6,687,249
Pass
Loans
Portfolio loans	6,078,957	5,997,126
Watch
Loans
Portfolio loans	370,337	413,876
Special Mention
Loans
Portfolio loans	192,470	181,107
Substandard
Loans
Portfolio loans	78,063	75,215
Substandard Non-accrual
Loans
Portfolio loans	25,672	27,896
Commercial
Loans
Portfolio loans	1,767,191
Commercial | Pass
Loans
Portfolio loans	1,481,538	1,458,416
Commercial | Watch
Loans
Portfolio loans	150,303	172,526
Commercial | Special Mention
Loans
Portfolio loans	85,231	66,337
Commercial | Substandard
Loans
Portfolio loans	42,727	41,273
Commercial | Substandard Non-accrual
Loans
Portfolio loans	7,392	9,096
Commercial real estate
Loans
Portfolio loans	2,825,003
Commercial real estate | Pass
Loans
Portfolio loans	2,509,301	2,477,398
Commercial real estate | Watch
Loans
Portfolio loans	178,151	186,963
Commercial real estate | Special Mention
Loans
Portfolio loans	100,365	105,487
Commercial real estate | Substandard
Loans
Portfolio loans	28,245	26,204
Commercial real estate | Substandard Non-accrual
Loans
Portfolio loans	8,941	9,178
Real estate construction
Loans
Portfolio loans	448,313
Real estate construction | Pass
Loans
Portfolio loans	415,302	351,923
Real estate construction | Watch
Loans
Portfolio loans	28,808	45,262
Real estate construction | Special Mention
Loans
Portfolio loans	3,222	3,928
Real estate construction | Substandard
Loans
Portfolio loans	699	737
Real estate construction | Substandard Non-accrual
Loans
Portfolio loans	282	630
Retail Real Estate
Loans
Portfolio loans	1,656,628
Retail Real Estate | Pass
Loans
Portfolio loans	1,624,543	1,661,691
Retail Real Estate | Watch
Loans
Portfolio loans	13,075	9,125
Retail Real Estate | Special Mention
Loans
Portfolio loans	3,652	5,355
Retail Real Estate | Substandard
Loans
Portfolio loans	6,387	7,001
Retail Real Estate | Substandard Non-accrual
Loans
Portfolio loans	8,971	8,935
Retail Other
Loans
Portfolio loans	48,364
Retail Other | Pass
Loans
Portfolio loans	48,273	47,698
Retail Other | Substandard
Loans
Portfolio loans	5
Retail Other | Substandard Non-accrual
Loans
Portfolio loans	86	57
Commercial Loans
Loans
Portfolio loans	1,767,191	1,748,368
Commercial Real Estate Loans
Loans
Portfolio loans	2,825,003	2,793,417
Real Estate Construction
Loans
Portfolio loans	448,313	401,861
Retail Real Estate
Loans
Portfolio loans	1,656,628	1,693,769
Retail Other
Loans
Portfolio loans	$ 48,364	$ 49,834