Fair Value Measurements - Fair Value Measurements at Reporting Date and Gain (Losses) from Fair Value Changes. (Details) - Non-Recurring - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired Loans	$ 4,617	$ 7,296
Other real estate owned	2,047	908
Gain (losses) from fair value changes, Impaired Loans	(332)	(234)
Gain (losses) from fair value changes, Other real estate owned	(532)	(76)
(Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired Loans	0	0
Other real estate owned	0	0
(Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired Loans	4,617	7,296
Other real estate owned	2,047	908
(Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired Loans	0	0
Other real estate owned	$ 0	$ 0