Investment securities - Summary of Amortized Cost of Securities and Fair Values (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Abstract]
Amortized cost	$ 668,033	$ 541,028
Gross unrealized gains	2,756
Gross unrealized losses	(15,091)
Available-for-sale debt securities, at fair value	655,698	536,270
Amortized cost		548,898
Gross unrealized gains		3,067
Gross unrealized losses		(7,973)
Fair Value		543,992
U.S. government agency securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	1,000
Gross unrealized gains	0
Gross unrealized losses	(11)
Available-for-sale debt securities, at fair value	989
Amortized cost		999
Gross unrealized gains		0
Gross unrealized losses		(13)
Fair Value		986
Mortgage-backed securities - residential
Debt Securities, Available-for-sale [Abstract]
Amortized cost	520,654
Gross unrealized gains	1,191
Gross unrealized losses	(13,265)
Available-for-sale debt securities, at fair value	508,580
Amortized cost		425,557
Gross unrealized gains		374
Gross unrealized losses		(7,150)
Fair Value		418,781
Municipals, tax exempt
Debt Securities, Available-for-sale [Abstract]
Amortized cost	138,994
Gross unrealized gains	1,565
Gross unrealized losses	(1,672)
Available-for-sale debt securities, at fair value	138,887
Amortized cost		107,127
Gross unrealized gains		2,692
Gross unrealized losses		(568)
Fair Value		109,251
Treasury securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost	7,385
Gross unrealized gains	0
Gross unrealized losses	(143)
Available-for-sale debt securities, at fair value	$ 7,242
Amortized cost		7,345
Gross unrealized gains		0
Gross unrealized losses		(93)
Fair Value		7,252
Debt Securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost		541,028
Gross unrealized gains		3,066
Gross unrealized losses		(7,824)
Fair Value		536,270
Equity securities
Debt Securities, Available-for-sale [Abstract]
Amortized cost		7,870
Gross unrealized gains		1
Gross unrealized losses		(149)
Fair Value		$ 7,722

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).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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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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-Name Accounting Standards Codification
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as 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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-Publisher FASB
-Name Accounting Standards Codification
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