Investment securities - Summary of Amortized Cost of Securities and Fair Values (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Abstract]
Amortized cost	$ 669,453	$ 668,033
Gross unrealized gains	6,311	2,756
Gross unrealized losses	(8,110)	(15,091)
Fair Value	667,654	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	(7)	(11)
Fair Value	993	989
Mortgage-backed securities - residential
Debt Securities, Available-for-sale [Abstract]
Amortized cost	516,917	520,654
Gross unrealized gains	2,407	1,191
Gross unrealized losses	(7,608)	(13,265)
Fair Value	511,716	508,580
Municipals, tax exempt
Debt Securities, Available-for-sale [Abstract]
Amortized cost	144,142	138,994
Gross unrealized gains	3,904	1,565
Gross unrealized losses	(406)	(1,672)
Fair Value	147,640	138,887
Treasury securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	7,394	7,385
Gross unrealized gains	0	0
Gross unrealized losses	(89)	(143)
Fair Value	$ 7,305	$ 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
-Name Accounting Standards Codification
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