Fair Value Measurements - Summary of Financial Instruments Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets
Securities available for sale	$ 670,328	$ 641,382
State tax credits held for sale	171	400
Portion at Other than Fair Value, Fair Value Disclosure
Assets
State tax credits held for sale	45,500
Estimate of Fair Value Measurement [Member]
Assets
Securities available for sale	670,328	641,382
State tax credits held for sale	48,622	44,271
Derivative financial instruments	4,895	3,589
Liabilities
Derivative financial instruments	4,895	3,589
Obligations of U.S. Government-sponsored enterprises
Assets
Securities available for sale	97,681	99,224
Obligations of states and political subdivisions
Assets
Securities available for sale	30,533	34,642
Residential mortgage-backed securities
Assets
Securities available for sale	532,347	507,516
US Treasury Bill Securities [Member]
Assets
Securities available for sale	9,767
Recurring basis \| Estimate of Fair Value Measurement [Member]
Assets
Securities available for sale	670,328	641,382
State tax credits held for sale	171	400
Other Investments	144
Derivative financial instruments	4,895	3,589
Total assets	675,538	645,371
Liabilities
Derivative financial instruments		3,589
Total liabilities	4,895	3,589
Recurring basis \| Obligations of U.S. Government-sponsored enterprises \| Estimate of Fair Value Measurement [Member]
Assets
Securities available for sale	97,681	99,224
Recurring basis \| Obligations of states and political subdivisions \| Estimate of Fair Value Measurement [Member]
Assets
Securities available for sale	30,533	34,642
Recurring basis \| Residential mortgage-backed securities \| Estimate of Fair Value Measurement [Member]
Assets
Securities available for sale	532,347	507,516
Recurring basis \| US Treasury Bill Securities [Member] \| Estimate of Fair Value Measurement [Member]
Assets
Securities available for sale	9,767
Recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Securities available for sale	0	0
State tax credits held for sale	0	0
Other Investments	144
Derivative financial instruments	0	0
Total assets	144	0
Liabilities
Derivative financial instruments	0	0
Total liabilities	0	0
Recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Obligations of U.S. Government-sponsored enterprises
Assets
Securities available for sale	0	0
Recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Obligations of states and political subdivisions
Assets
Securities available for sale	0	0
Recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Residential mortgage-backed securities
Assets
Securities available for sale	0	0
Recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| US Treasury Bill Securities [Member]
Assets
Securities available for sale	0
Recurring basis \| Significant Other Observable Inputs (Level 2)
Assets
Securities available for sale	670,328	641,382
State tax credits held for sale	0	0
Other Investments	0
Derivative financial instruments	4,895	3,589
Total assets	675,223	644,971
Liabilities
Derivative financial instruments	4,895	3,589
Total liabilities	4,895	3,589
Recurring basis \| Significant Other Observable Inputs (Level 2) \| Obligations of U.S. Government-sponsored enterprises
Assets
Securities available for sale	97,681	99,224
Recurring basis \| Significant Other Observable Inputs (Level 2) \| Obligations of states and political subdivisions
Assets
Securities available for sale	30,533	34,642
Recurring basis \| Significant Other Observable Inputs (Level 2) \| Residential mortgage-backed securities
Assets
Securities available for sale	532,347	507,516
Recurring basis \| Significant Other Observable Inputs (Level 2) \| US Treasury Bill Securities [Member]
Assets
Securities available for sale	9,767
Recurring basis \| Significant Unobservable Inputs (Level 3)
Assets
Securities available for sale	0	0
State tax credits held for sale	171	400
Other Investments	0
Derivative financial instruments	0	0
Total assets	171	400
Liabilities
Derivative financial instruments	0	0
Total liabilities	0	0
Recurring basis \| Significant Unobservable Inputs (Level 3) \| Obligations of U.S. Government-sponsored enterprises
Assets
Securities available for sale	0	0
Recurring basis \| Significant Unobservable Inputs (Level 3) \| Obligations of states and political subdivisions
Assets
Securities available for sale	0	0
Recurring basis \| Significant Unobservable Inputs (Level 3) \| Residential mortgage-backed securities
Assets
Securities available for sale	0	$ 0
Recurring basis \| Significant Unobservable Inputs (Level 3) \| US Treasury Bill Securities [Member]
Assets
Securities available for sale	$ 0

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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