Portfolio loans - Summary of risk grades segregated by category (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Loans
Portfolio loans	$ 7,229,020	$ 6,687,249
Pass
Loans
Portfolio loans	6,521,097	5,997,126
Watch
Loans
Portfolio loans	406,674	413,876
Special Mention
Loans
Portfolio loans	192,417	181,107
Substandard
Loans
Portfolio loans	83,737	75,215
Substandard Non-accrual
Loans
Portfolio loans	25,095	27,896
Commercial
Loans
Portfolio loans	2,357,954
Commercial | Pass
Loans
Portfolio loans	2,078,284	1,458,416
Commercial | Watch
Loans
Portfolio loans	136,817	172,526
Commercial | Special Mention
Loans
Portfolio loans	94,439	66,337
Commercial | Substandard
Loans
Portfolio loans	41,978	41,273
Commercial | Substandard Non-accrual
Loans
Portfolio loans	6,436	9,096
Commercial real estate
Loans
Portfolio loans	2,847,014
Commercial real estate | Pass
Loans
Portfolio loans	2,476,189	2,477,398
Commercial real estate | Watch
Loans
Portfolio loans	233,826	186,963
Commercial real estate | Special Mention
Loans
Portfolio loans	93,676	105,487
Commercial real estate | Substandard
Loans
Portfolio loans	34,314	26,204
Commercial real estate | Substandard Non-accrual
Loans
Portfolio loans	9,009	9,178
Real estate construction
Loans
Portfolio loans	433,031
Real estate construction | Pass
Loans
Portfolio loans	405,327	351,923
Real estate construction | Watch
Loans
Portfolio loans	23,907	45,262
Real estate construction | Special Mention
Loans
Portfolio loans	685	3,928
Real estate construction | Substandard
Loans
Portfolio loans	2,832	737
Real estate construction | Substandard Non-accrual
Loans
Portfolio loans	280	630
Retail Real Estate
Loans
Portfolio loans	1,548,215
Retail Real Estate | Pass
Loans
Portfolio loans	1,518,644	1,661,691
Retail Real Estate | Watch
Loans
Portfolio loans	12,124	9,125
Retail Real Estate | Special Mention
Loans
Portfolio loans	3,617	5,355
Retail Real Estate | Substandard
Loans
Portfolio loans	4,613	7,001
Retail Real Estate | Substandard Non-accrual
Loans
Portfolio loans	9,217	8,935
Retail Other
Loans
Portfolio loans	42,806
Retail Other | Pass
Loans
Portfolio loans	42,653	47,698
Retail Other | Substandard Non-accrual
Loans
Portfolio loans	153	57
Commercial Loans
Loans
Portfolio loans	2,357,954	1,748,368
Commercial Real Estate Loans
Loans
Portfolio loans	2,847,014	2,793,417
Real Estate Construction
Loans
Portfolio loans	433,031	401,861
Retail Real Estate
Loans
Portfolio loans	1,548,215	1,693,769
Retail Other
Loans
Portfolio loans	$ 42,806	$ 49,834