Portfolio loans - Distribution of portfolio loans (Details) - USD ($)	3 Months Ended		6 Months Ended	12 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Distributions of portfolio loans
Portfolio loans	$ 7,229,020,000		$ 7,229,020,000	$ 6,687,249,000
Allowance for loan losses	(96,046,000)	$ (84,384,000)	(96,046,000)	(53,748,000)	$ (51,375,000)	$ (50,915,000)	$ (50,648,000)
Portfolio loans, net	7,132,974,000		7,132,974,000	6,633,501,000
Net deferred loan origination costs	11,100,000		11,100,000	6,200,000
Net accretable purchase accounting adjustments			15,500,000	20,200,000
Commercial Loans
Distributions of portfolio loans
Portfolio loans	2,357,954,000		2,357,954,000	1,748,368,000
Allowance for loan losses	(24,146,000)	(22,725,000)	(24,146,000)	(18,291,000)	(16,733,000)	(17,998,000)	(17,829,000)
Commercial Real Estate Loans
Distributions of portfolio loans
Portfolio loans	2,847,014,000		2,847,014,000	2,793,417,000
Allowance for loan losses	(42,680,000)	(35,967,000)	(42,680,000)	(21,190,000)	(20,188,000)	(20,097,000)	(21,137,000)
Real Estate Construction
Distributions of portfolio loans
Portfolio loans	433,031,000		433,031,000	401,861,000
Allowance for loan losses	(7,792,000)	(7,193,000)	(7,792,000)	(3,204,000)	(3,305,000)	(2,807,000)	(2,723,000)
Retail Real Estate
Distributions of portfolio loans
Portfolio loans	1,548,215,000		1,548,215,000	1,693,769,000
Allowance for loan losses	(20,405,000)	(17,454,000)	(20,405,000)	(10,495,000)	(10,613,000)	(9,503,000)	(8,471,000)
Loans purchased	0	43,900,000
Retail Other
Distributions of portfolio loans
Portfolio loans	42,806,000		42,806,000	49,834,000
Allowance for loan losses	(1,023,000)	$ (1,045,000)	(1,023,000)	$ (568,000)	$ (536,000)	$ (510,000)	$ (488,000)
Paycheck Protection Program
Distributions of portfolio loans
Amortized cost of the loans outstanding	$ 729,300,000		$ 729,300,000