Other Noncurrent Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Liabilities, Other than Long-term Debt, Noncurrent [Abstract]
Tax amortization benefit contingency	$ 34,628	$ 31,562
Other	3,890	6,943
Total other noncurrent liabilities	$ 38,518	$ 38,505