Fair Value Measurement - Fair Value of Assets (Liabilities) Not Measured at Fair Value In Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Securities | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	$ 0	$ 0
Debt Securities | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	0	0
Debt Securities | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value	20,385	20,519
Long-term Debt | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long term debt, fair value	0	0
Long-term Debt | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long term debt, fair value	(273,476)	(368,408)
Long-term Debt | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long term debt, fair value	$ 0	$ 0