Loans and allowance for credit losses - Past Due Loans (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Loans	$ 7,213,538	$ 7,213,538	$ 4,409,642	$ 4,409,642
Collectively Evaluated
Financing Receivable, Past Due [Line Items]
Non-accrual amortized cost				25,282
Non-accrual with no related allowance		13,172
Non-accrual with related allowance		21,413
Related allowance		3,016
Interest income on non-accrual loans	524	729
Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	34,585	34,585		21,062
30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans	15,151	15,151		18,477
90 Days or More and Accruing Interest
Financing Receivable, Past Due [Line Items]
Loans	9,064	9,064		5,543
Financing Receivables Current
Financing Receivable, Past Due [Line Items]
Loans	7,154,738	7,154,738		4,307,408
Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans	1,417,671	1,417,671		1,034,036
Commercial and industrial | Collectively Evaluated
Financing Receivable, Past Due [Line Items]
Non-accrual amortized cost				5,586
Non-accrual with no related allowance		913
Non-accrual with related allowance		5,666
Related allowance		936
Interest income on non-accrual loans	287	304
Commercial and industrial | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	6,579	6,579		5,587
Commercial and industrial | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans	1,581	1,581		1,918
Commercial and industrial | 90 Days or More and Accruing Interest
Financing Receivable, Past Due [Line Items]
Loans	11	11		291
Commercial and industrial | Financing Receivables Current
Financing Receivable, Past Due [Line Items]
Loans	1,409,500	1,409,500		1,024,556
Construction
Financing Receivable, Past Due [Line Items]
Loans	1,190,878	1,190,878		551,101
Construction | Collectively Evaluated
Financing Receivable, Past Due [Line Items]
Non-accrual amortized cost				1,254
Non-accrual with no related allowance		980
Non-accrual with related allowance		1,038
Related allowance		145
Interest income on non-accrual loans	42	48
Construction | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	2,018	2,018		1,087
Construction | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans	4,346	4,346		1,021
Construction | 90 Days or More and Accruing Interest
Financing Receivable, Past Due [Line Items]
Loans	196	196		42
Construction | Financing Receivables Current
Financing Receivable, Past Due [Line Items]
Loans	1,184,318	1,184,318		546,067
Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Loans	1,140,611	1,140,611		710,454
Residential real estate: | 1-to-4 family mortgage | Collectively Evaluated
Financing Receivable, Past Due [Line Items]
Non-accrual amortized cost				4,585
Non-accrual with no related allowance		4,247
Non-accrual with related allowance		2,505
Related allowance		62
Interest income on non-accrual loans	15	21
Residential real estate: | 1-to-4 family mortgage | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	6,752	6,752		3,332
Residential real estate: | Residential line of credit
Financing Receivable, Past Due [Line Items]
Loans	420,318	420,318		221,530
Residential real estate: | Residential line of credit | Collectively Evaluated
Financing Receivable, Past Due [Line Items]
Non-accrual amortized cost				489
Non-accrual with no related allowance		0
Non-accrual with related allowance		1,925
Related allowance		74
Interest income on non-accrual loans	72	72
Residential real estate: | Residential line of credit | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	1,925	1,925		416
Residential real estate: | Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Loans	165,937	165,937		69,429
Residential real estate: | Multi-family mortgage | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	0	0		0
Residential real estate: | 30-89 Days Past Due | 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Loans	4,143	4,143		10,738
Residential real estate: | 30-89 Days Past Due | Residential line of credit
Financing Receivable, Past Due [Line Items]
Loans	287	287		658
Residential real estate: | 30-89 Days Past Due | Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Loans	0	0		63
Residential real estate: | 90 Days or More and Accruing Interest | 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Loans	7,452	7,452		3,965
Residential real estate: | 90 Days or More and Accruing Interest | Residential line of credit
Financing Receivable, Past Due [Line Items]
Loans	216	216		412
Residential real estate: | 90 Days or More and Accruing Interest | Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Loans	57	57		0
Residential real estate: | Financing Receivables Current | 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Loans	1,122,264	1,122,264		673,081
Residential real estate: | Financing Receivables Current | Residential line of credit
Financing Receivable, Past Due [Line Items]
Loans	417,890	417,890		219,971
Residential real estate: | Financing Receivables Current | Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Loans	165,880	165,880		69,366
Commercial real estate: | Owner occupied
Financing Receivable, Past Due [Line Items]
Loans	924,987	924,987		630,270
Commercial real estate: | Owner occupied | Collectively Evaluated
Financing Receivable, Past Due [Line Items]
Non-accrual amortized cost				2,285
Non-accrual with no related allowance		1,469
Non-accrual with related allowance		725
Related allowance		66
Interest income on non-accrual loans	32	75
Commercial real estate: | Owner occupied | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	2,194	2,194		1,793
Commercial real estate: | Non-owner occupied
Financing Receivable, Past Due [Line Items]
Loans	1,644,400	1,644,400		920,744
Commercial real estate: | Non-owner occupied | Collectively Evaluated
Financing Receivable, Past Due [Line Items]
Non-accrual amortized cost				9,460
Non-accrual with no related allowance		5,475
Non-accrual with related allowance		6,883
Related allowance		1,568
Interest income on non-accrual loans	76	185
Commercial real estate: | Non-owner occupied | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	12,358	12,358		7,880
Commercial real estate: | 30-89 Days Past Due | Owner occupied
Financing Receivable, Past Due [Line Items]
Loans	1,096	1,096		1,375
Commercial real estate: | 30-89 Days Past Due | Non-owner occupied
Financing Receivable, Past Due [Line Items]
Loans	114	114		327
Commercial real estate: | 90 Days or More and Accruing Interest | Owner occupied
Financing Receivable, Past Due [Line Items]
Loans	0	0		0
Commercial real estate: | 90 Days or More and Accruing Interest | Non-owner occupied
Financing Receivable, Past Due [Line Items]
Loans	0	0		0
Commercial real estate: | Financing Receivables Current | Owner occupied
Financing Receivable, Past Due [Line Items]
Loans	921,697	921,697		621,211
Commercial real estate: | Financing Receivables Current | Non-owner occupied
Financing Receivable, Past Due [Line Items]
Loans	1,631,928	1,631,928		902,340
Consumer and other
Financing Receivable, Past Due [Line Items]
Loans	308,736	308,736		272,078
Consumer and other | Collectively Evaluated
Financing Receivable, Past Due [Line Items]
Non-accrual amortized cost				1,623
Non-accrual with no related allowance		88
Non-accrual with related allowance		2,671
Related allowance		165
Interest income on non-accrual loans	0	24
Consumer and other | Non Accruing
Financing Receivable, Past Due [Line Items]
Loans	2,759	2,759		967
Consumer and other | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans	3,584	3,584		2,377
Consumer and other | 90 Days or More and Accruing Interest
Financing Receivable, Past Due [Line Items]
Loans	1,132	1,132		833
Consumer and other | Financing Receivables Current
Financing Receivable, Past Due [Line Items]
Loans	$ 301,261	$ 301,261		250,816
Purchased Credit Impaired
Financing Receivable, Past Due [Line Items]
Loans				57,152
Purchased Credit Impaired | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans				1,684
Purchased Credit Impaired | Construction
Financing Receivable, Past Due [Line Items]
Loans				2,884
Purchased Credit Impaired | Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Loans				19,338
Purchased Credit Impaired | Residential real estate: | Residential line of credit
Financing Receivable, Past Due [Line Items]
Loans				73
Purchased Credit Impaired | Residential real estate: | Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Loans				0
Purchased Credit Impaired | Commercial real estate: | Owner occupied
Financing Receivable, Past Due [Line Items]
Loans				5,891
Purchased Credit Impaired | Commercial real estate: | Non-owner occupied
Financing Receivable, Past Due [Line Items]
Loans				10,197
Purchased Credit Impaired | Consumer and other
Financing Receivable, Past Due [Line Items]
Loans				$ 17,085