Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Carrying Value
Fair Value of Financial Instruments
Liabilities - long-term debt (carried at cost)	$ 309,316	$ 284,699
Fair Value
Fair Value of Financial Instruments
Liabilities - long-term debt (carried at cost)	315,339	293,765
Recurring
Fair Value of Financial Instruments
Liabilities - long-term debt (carried at cost)	315,339	293,765
Significant other observable inputs (Level 2) | Recurring
Fair Value of Financial Instruments
Liabilities - long-term debt (carried at cost)	$ 315,339	$ 293,765