Investment securities - Summary of Amortized Cost of Securities and Fair Values (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Abstract]
Amortized cost	$ 1,129,053,000	$ 676,705,000
Gross unrealized gains	32,503,000	13,567,000
Gross unrealized losses	(1,035,000)	(1,891,000)
Allowance for credit losses for investments	0
Fair Value	1,160,521,000	688,381,000
U.S. government agency securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	2,000,000
Gross unrealized gains	0
Gross unrealized losses	(6,000)
Allowance for credit losses for investments	0
Fair Value	1,994,000
Mortgage-backed securities - residential
Debt Securities, Available-for-sale [Abstract]
Amortized cost	723,333,000	474,144,000
Gross unrealized gains	15,093,000	4,829,000
Gross unrealized losses	(320,000)	(1,661,000)
Allowance for credit losses for investments	0
Fair Value	738,106,000	477,312,000
Mortgage-backed securities - commercial
Debt Securities, Available-for-sale [Abstract]
Amortized cost	20,478,000	12,957,000
Gross unrealized gains	1,376,000	407,000
Gross unrealized losses	0	0
Allowance for credit losses for investments	0
Fair Value	21,854,000	13,364,000
Municipals, tax exempt
Debt Securities, Available-for-sale [Abstract]
Amortized cost	359,761,000	181,178,000
Gross unrealized gains	15,813,000	8,287,000
Gross unrealized losses	(694,000)	(230,000)
Allowance for credit losses for investments	0
Fair Value	374,880,000	189,235,000
Treasury securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	21,482,000	7,426,000
Gross unrealized gains	218,000	22,000
Gross unrealized losses	0	0
Allowance for credit losses for investments	0
Fair Value	21,700,000	7,448,000
Corporate securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	1,999,000	1,000,000
Gross unrealized gains	3,000	22,000
Gross unrealized losses	(15,000)	0
Allowance for credit losses for investments	0
Fair Value	$ 1,987,000	$ 1,022,000