Fair Values of Financial Instruments - Reconciliation of Beginning and Ending Balances of Recurring Fair Value Measurements using Significant Unobservable Level 3 Inputs (Details) - Fair Value, Measurements, Recurring - Available for Sale Securities - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Available for Sale Securities
Beginning Balance	$ 5,169	$ 5,932
Included in other comprehensive income	26	(223)
Principal payments	(1,217)	(540)
Ending balance	$ 3,978	$ 5,169