Portfolio loans - Summary of risk grades segregated by category (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Loans
Portfolio loans	$ 7,121,311	$ 6,687,249
Pass
Loans
Portfolio loans	6,421,496	5,997,126
Watch
Loans
Portfolio loans	439,044	413,876
Special Mention
Loans
Portfolio loans	158,929	181,107
Substandard
Loans
Portfolio loans	77,944	75,215
Substandard Non-accrual
Loans
Portfolio loans	23,898	27,896
Commercial
Loans
Portfolio loans	2,281,031
Commercial | Pass
Loans
Portfolio loans	2,011,113	1,458,416
Commercial | Watch
Loans
Portfolio loans	145,817	172,526
Commercial | Special Mention
Loans
Portfolio loans	82,942	66,337
Commercial | Substandard
Loans
Portfolio loans	33,247	41,273
Commercial | Substandard Non-accrual
Loans
Portfolio loans	7,912	9,096
Commercial real estate
Loans
Portfolio loans	2,912,408
Commercial real estate | Pass
Loans
Portfolio loans	2,541,455	2,477,398
Commercial real estate | Watch
Loans
Portfolio loans	255,447	186,963
Commercial real estate | Special Mention
Loans
Portfolio loans	71,315	105,487
Commercial real estate | Substandard
Loans
Portfolio loans	37,052	26,204
Commercial real estate | Substandard Non-accrual
Loans
Portfolio loans	7,139	9,178
Real estate construction
Loans
Portfolio loans	407,266
Real estate construction | Pass
Loans
Portfolio loans	376,787	351,923
Real estate construction | Watch
Loans
Portfolio loans	26,820	45,262
Real estate construction | Special Mention
Loans
Portfolio loans	603	3,928
Real estate construction | Substandard
Loans
Portfolio loans	2,797	737
Real estate construction | Substandard Non-accrual
Loans
Portfolio loans	259	630
Retail Real Estate
Loans
Portfolio loans	1,480,520
Retail Real Estate | Pass
Loans
Portfolio loans	1,452,145	1,661,691
Retail Real Estate | Watch
Loans
Portfolio loans	10,960	9,125
Retail Real Estate | Special Mention
Loans
Portfolio loans	4,069	5,355
Retail Real Estate | Substandard
Loans
Portfolio loans	4,848	7,001
Retail Real Estate | Substandard Non-accrual
Loans
Portfolio loans	8,498	8,935
Retail Other
Loans
Portfolio loans	40,086
Retail Other | Pass
Loans
Portfolio loans	39,996	47,698
Retail Other | Substandard Non-accrual
Loans
Portfolio loans	90	57
Commercial Loans
Loans
Portfolio loans	2,281,031	1,748,368
Commercial Real Estate Loans
Loans
Portfolio loans	2,912,408	2,793,417
Real Estate Construction
Loans
Portfolio loans	407,266	401,861
Retail Real Estate
Loans
Portfolio loans	1,480,520	1,693,769
Retail Other
Loans
Portfolio loans	$ 40,086	$ 49,834