Investment securities - Summary of Amortized Cost of Securities and Fair Values (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Abstract]
Amortized cost	$ 736,172,000	$ 676,705,000
Gross unrealized gains	28,440,000	13,567,000
Gross unrealized losses	(395,000)	(1,891,000)
Allowance for credit losses for investments	0
Fair Value	764,217,000	688,381,000
U.S. government agency securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	3,007,000
Gross unrealized gains	30,000
Gross unrealized losses	0
Allowance for credit losses for investments	0
Fair Value	3,037,000
Mortgage-backed securities - residential
Debt Securities, Available-for-sale [Abstract]
Amortized cost	481,651,000	487,101,000
Gross unrealized gains	18,028,000	5,236,000
Gross unrealized losses	(21,000)	(1,661,000)
Allowance for credit losses for investments	0
Fair Value	499,658,000	490,676,000
Municipals, tax exempt
Debt Securities, Available-for-sale [Abstract]
Amortized cost	226,026,000	181,178,000
Gross unrealized gains	10,010,000	8,287,000
Gross unrealized losses	(359,000)	(230,000)
Allowance for credit losses for investments	0
Fair Value	235,677,000	189,235,000
Treasury securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	24,488,000	7,426,000
Gross unrealized gains	372,000	22,000
Gross unrealized losses	0	0
Allowance for credit losses for investments	0
Fair Value	24,860,000	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