Business combinations - Pre-tax merger-related costs (Details) - USD ($)	3 Months Ended							12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Salaries and employee benefits								$ 6,580,000	$ 3,569,000
Other								4,500,000	5,598,000	$ 671,000
Pre-tax acquisition costs	$ 9,879,000	$ 661,000	$ 540,000	$ 139,000	$ 199,000	$ 8,214,000	$ 615,000	11,080,000	9,167,000	671,000
Presidio bank
Business Acquisition [Line Items]
Salaries and employee benefits								6,580,000
Other								4,500,000
Pre-tax acquisition costs								$ 11,080,000
Tri Valley Bank
Business Acquisition [Line Items]
Salaries and employee benefits									3,569,000
Other									5,598,000
Pre-tax acquisition costs									$ 9,167,000
United American Bank
Business Acquisition [Line Items]
Other										671,000
Pre-tax acquisition costs										$ 671,000