Investment Securities - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 562,528	$ 745,328
Gross Unrealized Gains	1,655	805
Gross Unrealized Losses	(4,200)	(8,691)
Fair Value	559,983	737,442
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	100	200,026
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(13)
Fair Value	100	200,013
Obligations of the U.S. government and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	188,079	198,604
Gross Unrealized Gains	103	107
Gross Unrealized Losses	(1,436)	(2,856)
Fair Value	186,746	195,855
Obligations of state and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,644	11,372
Gross Unrealized Gains	5	3
Gross Unrealized Losses	(11)	(43)
Fair Value	8,638	11,332
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	323,610	294,076
Gross Unrealized Gains	1,365	554
Gross Unrealized Losses	(2,062)	(4,740)
Fair Value	322,913	289,890
Collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	40,995	40,150
Gross Unrealized Gains	182	141
Gross Unrealized Losses	(691)	(1,039)
Fair Value	40,486	39,252
Other investment securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,100	1,100
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 1,100	$ 1,100

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).

+ References

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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
-Name Accounting Standards Codification
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-Section 50
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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 45
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