Investment securities - Summary of Amortized Cost of Securities and Fair Values (Details) - USD ($)	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Abstract]
Amortized cost	$ 717,755,000	$ 676,705,000
Gross unrealized gains	29,952,000	13,567,000
Gross unrealized losses	(269,000)	(1,891,000)
Allowance for credit losses for investments	0
Fair Value	747,438,000	688,381,000
U.S. government agency securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	3,003,000
Gross unrealized gains	21,000
Gross unrealized losses	0
Allowance for credit losses for investments	0
Fair Value	3,024,000
Mortgage-backed securities - residential
Debt Securities, Available-for-sale [Abstract]
Amortized cost	439,843,000	487,101,000
Gross unrealized gains	14,770,000	5,236,000
Gross unrealized losses	(7,000)	(1,661,000)
Allowance for credit losses for investments	0
Fair Value	454,606,000	490,676,000
Municipals, tax exempt
Debt Securities, Available-for-sale [Abstract]
Amortized cost	251,424,000	181,178,000
Gross unrealized gains	14,875,000	8,287,000
Gross unrealized losses	(247,000)	(230,000)
Allowance for credit losses for investments	0
Fair Value	266,052,000	189,235,000
Treasury securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	22,485,000	7,426,000
Gross unrealized gains	286,000	22,000
Gross unrealized losses	0	0
Allowance for credit losses for investments	0
Fair Value	22,771,000	7,448,000
Corporate securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	1,000,000	1,000,000
Gross unrealized gains	0	22,000
Gross unrealized losses	(15,000)	0
Allowance for credit losses for investments	0
Fair Value	$ 985,000	$ 1,022,000