Investment securities - Summary of Amortized Cost of Securities and Fair Values (Details) - USD ($)	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Debt Securities, Available-for-sale [Abstract]
Amortized cost	$ 1,205,933,000	$ 1,137,848,000
Gross unrealized gains	26,339,000	35,376,000
Gross unrealized losses	(7,094,000)	(824,000)
Allowance for credit losses for investments	0	0	$ 0
Fair Value	1,225,178,000	1,172,400,000
U.S. government agency securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost		2,000,000
Gross unrealized gains		3,000
Gross unrealized losses		0
Allowance for credit losses for investments		0
Fair Value		2,003,000
Mortgage-backed securities - residential
Debt Securities, Available-for-sale [Abstract]
Amortized cost	834,335,000	760,099,000
Gross unrealized gains	10,971,000	14,040,000
Gross unrealized losses	(6,598,000)	(803,000)
Allowance for credit losses for investments	0	0
Fair Value	838,708,000	773,336,000
Mortgage-backed securities - commercial
Debt Securities, Available-for-sale [Abstract]
Amortized cost	19,888,000	20,226,000
Gross unrealized gains	757,000	1,362,000
Gross unrealized losses	(10,000)	0
Allowance for credit losses for investments	0	0
Fair Value	20,635,000	21,588,000
Municipal securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	334,730,000	336,543,000
Gross unrealized gains	14,515,000	19,806,000
Gross unrealized losses	(469,000)	(20,000)
Allowance for credit losses for investments	0	0
Fair Value	348,776,000	356,329,000
U.S. Treasury securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	14,480,000	16,480,000
Gross unrealized gains	96,000	148,000
Gross unrealized losses	0	0
Allowance for credit losses for investments	0	0
Fair Value	14,576,000	16,628,000
Corporate securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	2,500,000	2,500,000
Gross unrealized gains	0	17,000
Gross unrealized losses	(17,000)	(1,000)
Allowance for credit losses for investments	0	0
Fair Value	$ 2,483,000	$ 2,516,000