Fair Value Accounting - Carrying Values and Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities	$ 11,795	$ 36,488
Senior notes	85,938	85,938
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities	11,795	36,488
Senior notes	$ 23,018	$ 23,349
Post-Modification Notes | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Basis spread on variable rate	3.50%