Loans and allowance for credit losses - Allowance for Loan Losses by Portfolio Segment and Related Investment in Loans Net of Unearned Interest (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	$ 89,141	$ 29,814	$ 31,139	$ 28,932
Provision for credit losses	24,039	881	52,003	2,272
Recoveries of loans previously charged-off	1,114	213	1,448	575
Loans charged off	(1,165)	(770)	(3,576)	(1,641)
Initial allowance on loans purchased with deteriorated credit quality			669
Balance at end of period	113,129	30,138	113,129	30,138
Commercial and industrial
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	10,881	5,514	4,805	5,348
Provision for credit losses	(2,663)	(550)	(834)	(217)
Recoveries of loans previously charged-off	807	38	895	50
Loans charged off	(147)	(79)	(1,381)	(258)
Initial allowance on loans purchased with deteriorated credit quality			11
Balance at end of period	8,878	4,923	8,878	4,923
Construction
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	22,842	9,758	10,194	9,729
Provision for credit losses	12,624	(109)	23,578	(81)
Recoveries of loans previously charged-off	151	6	151	7
Loans charged off	(18)	0	(18)	0
Initial allowance on loans purchased with deteriorated credit quality			11
Balance at end of period	35,599	9,655	35,599	9,655
Residential real estate: | 1-to-4 family mortgage
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	13,006	3,295	3,112	3,428
Provision for credit losses	(446)	(30)	1,218	(95)
Recoveries of loans previously charged-off	26	24	50	37
Loans charged off	(123)	(1)	(365)	(82)
Initial allowance on loans purchased with deteriorated credit quality			107
Balance at end of period	12,463	3,288	12,463	3,288
Residential real estate: | Residential line of credit
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	6,213	731	752	811
Provision for credit losses	595	106	2,580	33
Recoveries of loans previously charged-off	24	21	39	46
Loans charged off	(21)	(103)	(21)	(135)
Initial allowance on loans purchased with deteriorated credit quality			3
Balance at end of period	6,811	755	6,811	755
Residential real estate: | Multi-family mortgage
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	2,328	539	544	566
Provision for credit losses	2,171	78	3,615	51
Recoveries of loans previously charged-off	0	0	0	0
Loans charged off	0	0	0	0
Initial allowance on loans purchased with deteriorated credit quality			0
Balance at end of period	4,499	617	4,499	617
Commercial real estate: | Owner occupied
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	9,047	3,098	4,109	3,132
Provision for credit losses	(1,630)	409	1,408	288
Recoveries of loans previously charged-off	3	5	17	92
Loans charged off	0	0	(209)	0
Initial allowance on loans purchased with deteriorated credit quality			54
Balance at end of period	7,420	3,512	7,420	3,512
Commercial real estate: | Non-owner occupied
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	18,005	4,583	4,621	4,149
Provision for credit losses	12,984	(105)	18,919	329
Recoveries of loans previously charged-off	0	0	0	0
Loans charged off	(545)	0	(545)	0
Initial allowance on loans purchased with deteriorated credit quality			443
Balance at end of period	30,444	4,478	30,444	4,478
Consumer and other
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	6,819	2,296	3,002	1,769
Provision for credit losses	404	1,082	1,519	1,964
Recoveries of loans previously charged-off	103	119	296	343
Loans charged off	(311)	(587)	(1,037)	(1,166)
Initial allowance on loans purchased with deteriorated credit quality			40
Balance at end of period	$ 7,015	$ 2,910	7,015	$ 2,910
Non-purchased credit deteriorated loan | Accounting Standards Update 2019-11
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period			30,888
Non-purchased credit deteriorated loan | Commercial and industrial | Accounting Standards Update 2019-11
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period			5,300
Non-purchased credit deteriorated loan | Construction | Accounting Standards Update 2019-11
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period			1,533
Non-purchased credit deteriorated loan | Residential real estate: | 1-to-4 family mortgage | Accounting Standards Update 2019-11
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period			7,920
Non-purchased credit deteriorated loan | Residential real estate: | Residential line of credit | Accounting Standards Update 2019-11
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period			3,461
Non-purchased credit deteriorated loan | Residential real estate: | Multi-family mortgage | Accounting Standards Update 2019-11
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period			340
Non-purchased credit deteriorated loan | Commercial real estate: | Owner occupied | Accounting Standards Update 2019-11
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period			1,879
Non-purchased credit deteriorated loan | Commercial real estate: | Non-owner occupied | Accounting Standards Update 2019-11
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period			6,822
Non-purchased credit deteriorated loan | Consumer and other | Accounting Standards Update 2019-11
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period			3,633
Purchased credit deteriorated loan | Accounting Standards Update 2019-11
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period			558
Purchased credit deteriorated loan | Commercial and industrial | Accounting Standards Update 2019-11
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period			82
Purchased credit deteriorated loan | Construction | Accounting Standards Update 2019-11
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period			150
Purchased credit deteriorated loan | Residential real estate: | 1-to-4 family mortgage | Accounting Standards Update 2019-11
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period			421
Purchased credit deteriorated loan | Residential real estate: | Residential line of credit | Accounting Standards Update 2019-11
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period			(3)
Purchased credit deteriorated loan | Residential real estate: | Multi-family mortgage | Accounting Standards Update 2019-11
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period			0
Purchased credit deteriorated loan | Commercial real estate: | Owner occupied | Accounting Standards Update 2019-11
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period			162
Purchased credit deteriorated loan | Commercial real estate: | Non-owner occupied | Accounting Standards Update 2019-11
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period			184
Purchased credit deteriorated loan | Consumer and other | Accounting Standards Update 2019-11
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period			$ (438)