Investment securities - Summary of Amortized Cost of Securities and Fair Values (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Abstract]
Amortized cost	$ 676,705	$ 668,033
Gross unrealized gains	13,567	2,756
Gross unrealized losses	(1,891)	(15,091)
Fair Value	688,381	655,698
U.S. government agency securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost		1,000
Gross unrealized gains		0
Gross unrealized losses		(11)
Fair Value		989
Mortgage-backed securities - residential
Debt Securities, Available-for-sale [Abstract]
Amortized cost	487,101	520,654
Gross unrealized gains	5,236	1,191
Gross unrealized losses	(1,661)	(13,265)
Fair Value	490,676	508,580
Municipals, tax exempt
Debt Securities, Available-for-sale [Abstract]
Amortized cost	181,178	138,994
Gross unrealized gains	8,287	1,565
Gross unrealized losses	(230)	(1,672)
Fair Value	189,235	138,887
Treasury securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	7,426	7,385
Gross unrealized gains	22	0
Gross unrealized losses	0	(143)
Fair Value	7,448	$ 7,242
Corporate securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	1,000
Gross unrealized gains	22
Gross unrealized losses	0
Fair Value	$ 1,022

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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