Investment Securities - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 543,240	$ 692,824
Gross Unrealized Gains	362	1,316
Gross Unrealized Losses	(15,538)	(4,938)
Fair Value	528,064	689,202
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	100	200,077
Gross Unrealized Gains	0	11
Gross Unrealized Losses	0	0
Fair Value	100	200,088
Obligations of the U.S. government and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	196,334	153,028
Gross Unrealized Gains	17	75
Gross Unrealized Losses	(5,898)	(2,059)
Fair Value	190,453	151,044
Obligations of state and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	15,890	21,352
Gross Unrealized Gains	1	11
Gross Unrealized Losses	(92)	(53)
Fair Value	15,799	21,310
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	292,005	275,958
Gross Unrealized Gains	344	887
Gross Unrealized Losses	(7,928)	(1,855)
Fair Value	284,421	274,990
Collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	37,811	37,596
Gross Unrealized Gains	0	14
Gross Unrealized Losses	(1,618)	(948)
Fair Value	36,193	36,662
Other investment securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,100	4,813
Gross Unrealized Gains	0	318
Gross Unrealized Losses	(2)	(23)
Fair Value	$ 1,098	$ 5,108

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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-Publisher FASB
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-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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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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