Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Recurring
Fair Value of Financial Instruments
Assets - investment in U.S. Treasury obligations	$ 810	$ 808
Liabilities - long-term debt (carried at cost)	296,111	296,908
Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Fair Value of Financial Instruments
Assets - investment in U.S. Treasury obligations	810	808
Significant other observable inputs (Level 2) | Recurring
Fair Value of Financial Instruments
Liabilities - long-term debt (carried at cost)	296,111	296,908
Carrying Value
Fair Value of Financial Instruments
Assets - investment in U.S. Treasury obligations	810	808
Liabilities - long-term debt (carried at cost)	291,307	292,423
Fair Value
Fair Value of Financial Instruments
Assets - investment in U.S. Treasury obligations	810	808
Liabilities - long-term debt (carried at cost)	$ 296,111	$ 296,908