Financial Instruments and Fair Value Measurements - Carrying Amount and Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes held by special purpose entity	$ 176,595	$ 176,537
Carrying Value | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Time deposit	200,000	200,000
Senior notes held by special purpose entity	176,595	176,537
Carrying Value | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
U.S. Treasury securities and cash equivalents	7,618	7,989
Fair Value | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Time deposit	200,000	200,000
Senior notes held by special purpose entity	191,373	198,530
Fair Value | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
U.S. Treasury securities and cash equivalents	$ 7,303	$ 7,797