Loans and allowance for credit losses - Amount of Loans by Loan Category Broken Out Between Loans Individually Evaluated for Impairment and Collectively Evaluated for Impairment (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment			$ 23,810
Collectively evaluated for impairment			4,328,680
Fair value			57,152
Loans	$ 7,213,538	$ 4,409,642	4,409,642
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment			9,026
Collectively evaluated for impairment			1,023,326
Fair value			1,684
Loans	1,417,671		1,034,036
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment			2,061
Collectively evaluated for impairment			546,156
Fair value			2,884
Loans	1,190,878		551,101
Residential real estate: | 1-to-4 family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment			1,347
Collectively evaluated for impairment			689,769
Fair value			19,338
Loans	1,140,611		710,454
Residential real estate: | Residential line of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment			579
Collectively evaluated for impairment			220,878
Fair value			73
Loans	420,318		221,530
Residential real estate: | Multi-family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment			0
Collectively evaluated for impairment			69,429
Fair value			0
Loans	165,937		69,429
Commercial real estate: | Owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment			2,993
Collectively evaluated for impairment			621,386
Fair value			5,891
Loans	924,987		630,270
Commercial real estate: | Non-owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment			7,755
Collectively evaluated for impairment			902,792
Fair value			10,197
Loans	1,644,400		920,744
Consumer and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment			49
Collectively evaluated for impairment			254,944
Fair value			17,085
Loans	$ 308,736		$ 272,078