Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Carrying Value
Fair Value of Financial Instruments
Liabilities - long-term debt (carried at cost)	$ 305,054	$ 309,316
Fair Value
Fair Value of Financial Instruments
Liabilities - long-term debt (carried at cost)	310,709	315,339
Recurring
Fair Value of Financial Instruments
Liabilities - long-term debt (carried at cost)	310,709	315,339
Significant other observable inputs (Level 2) | Recurring
Fair Value of Financial Instruments
Liabilities - long-term debt (carried at cost)	$ 310,709	$ 315,339