Fair Values of Financial Instruments - Valuation Methodologies Used for Instruments Measured at Fair Value on Non-Recurring Basis (Details) - Fair Value, Measurements, Nonrecurring - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Significant Unobservable Inputs (Level 3) | Impaired Loans (collateral dependent)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset fair value	$ 9,576	$ 15,318
Significant Unobservable Inputs (Level 3) | Other real estate owned
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset fair value	859	1,612
Fair Value | Impaired Loans (collateral dependent)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset fair value	9,576	15,318
Fair Value | Other real estate owned
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset fair value	$ 859	$ 1,612