Investment securities - Summary of Amortized Cost of Securities and Fair Values (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Abstract]
Amortized cost	$ 656,095	$ 668,033
Gross unrealized gains	14,570	2,756
Gross unrealized losses	(2,134)	(15,091)
Fair Value	668,531	655,698
U.S. government agency securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	1,000	1,000
Gross unrealized gains	0	0
Gross unrealized losses	(1)	(11)
Fair Value	999	989
Mortgage-backed securities - residential
Debt Securities, Available-for-sale [Abstract]
Amortized cost	481,580	520,654
Gross unrealized gains	5,784	1,191
Gross unrealized losses	(2,064)	(13,265)
Fair Value	485,300	508,580
Municipals, tax exempt
Debt Securities, Available-for-sale [Abstract]
Amortized cost	165,100	138,994
Gross unrealized gains	8,754	1,565
Gross unrealized losses	(69)	(1,672)
Fair Value	173,785	138,887
Treasury securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	7,415	7,385
Gross unrealized gains	17	0
Gross unrealized losses	0	(143)
Fair Value	7,432	$ 7,242
Corporate securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	1,000
Gross unrealized gains	15
Gross unrealized losses	0
Fair Value	$ 1,015

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).

+ References

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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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