Investment securities - Summary of Amortized Cost of Securities and Fair Values (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Abstract]
Amortized cost	$ 667,912	$ 668,033
Gross unrealized gains	11,109	2,756
Gross unrealized losses	(3,806)	(15,091)
Fair Value	675,215	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	(4)	(11)
Fair Value	996	989
Mortgage-backed securities - residential
Debt Securities, Available-for-sale [Abstract]
Amortized cost	516,458	520,654
Gross unrealized gains	4,786	1,191
Gross unrealized losses	(3,739)	(13,265)
Fair Value	517,505	508,580
Municipals, tax exempt
Debt Securities, Available-for-sale [Abstract]
Amortized cost	143,049	138,994
Gross unrealized gains	6,319	1,565
Gross unrealized losses	(63)	(1,672)
Fair Value	149,305	138,887
Treasury securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	7,405	7,385
Gross unrealized gains	4	0
Gross unrealized losses	0	(143)
Fair Value	$ 7,409	$ 7,242

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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-Publisher FASB
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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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-Publisher FASB
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