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		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	$ 113,129	$ 30,138	$ 31,139	$ 28,932
Provision for credit losses	45,834	1,831	97,837	4,103
Recoveries of loans previously charged-off	1,172	212	2,620	787
Loans charged off	(993)	(717)	(4,569)	(2,358)
Initial allowance on loans purchased with deteriorated credit quality	24,831		25,500
Balance at end of period	183,973	31,464	183,973	31,464
Commercial and industrial
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	8,878	4,923	4,805	5,348
Provision for credit losses	(1,520)	234	(2,354)	17
Recoveries of loans previously charged-off	757	16	1,652	66
Loans charged off	(249)	(3)	(1,630)	(261)
Initial allowance on loans purchased with deteriorated credit quality	743		754
Balance at end of period	8,609	5,170	8,609	5,170
Construction
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	35,599	9,655	10,194	9,729
Provision for credit losses	22,383	186	45,962	105
Recoveries of loans previously charged-off	51	1	202	8
Loans charged off	0	0	(18)	0
Initial allowance on loans purchased with deteriorated credit quality	5,596		5,606
Balance at end of period	63,629	9,842	63,629	9,842
Residential real estate: | 1-to-4 family mortgage
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	12,463	3,288	3,112	3,428
Provision for credit losses	4,194	18	5,412	(77)
Recoveries of loans previously charged-off	116	25	166	62
Loans charged off	(8)	0	(373)	(82)
Initial allowance on loans purchased with deteriorated credit quality	1,533		1,640
Balance at end of period	18,298	3,331	18,298	3,331
Residential real estate: | Residential line of credit
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	6,811	755	752	811
Provision for credit losses	4,053	67	6,633	100
Recoveries of loans previously charged-off	22	75	61	121
Loans charged off	0	(170)	(21)	(305)
Initial allowance on loans purchased with deteriorated credit quality	569		572
Balance at end of period	11,455	727	11,455	727
Residential real estate: | Multi-family mortgage
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	4,499	617	544	566
Provision for credit losses	1,908	(43)	5,523	8
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	784		784
Balance at end of period	7,191	574	7,191	574
Commercial real estate: | Owner occupied
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	7,420	3,512	4,109	3,132
Provision for credit losses	(1,276)	194	132	482
Recoveries of loans previously charged-off	51	3	68	95
Loans charged off	(95)	0	(304)	0
Initial allowance on loans purchased with deteriorated credit quality	605		659
Balance at end of period	6,705	3,709	6,705	3,709
Commercial real estate: | Non-owner occupied
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	30,444	4,478	4,621	4,149
Provision for credit losses	12,364	461	31,282	790
Recoveries of loans previously charged-off	0	0	0	0
Loans charged off	(166)	(12)	(711)	(12)
Initial allowance on loans purchased with deteriorated credit quality	14,998		15,442
Balance at end of period	57,640	4,927	57,640	4,927
Consumer and other
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	7,015	2,910	3,002	1,769
Provision for credit losses	3,728	714	5,247	2,678
Recoveries of loans previously charged-off	175	92	471	435
Loans charged off	(475)	(532)	(1,512)	(1,698)
Initial allowance on loans purchased with deteriorated credit quality	3		43
Balance at end of period	$ 10,446	$ 3,184	10,446	$ 3,184
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)