Available-for-Sale Securities and Fair Value Measurements - Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Recurring basis
Assets
Total assets measured at estimated fair value	$ 50,546	$ 122,709
Recurring basis | U.S. government securities
Assets
Total assets measured at estimated fair value		9,942
Recurring basis | U.S. government agencies
Assets
Total assets measured at estimated fair value		9,985
Recurring basis | U.S. corporate notes
Assets
Total assets measured at estimated fair value	3,873	10,866
Recurring basis | U.S. commercial papers
Assets
Total assets measured at estimated fair value	7,988	29,945
Recurring basis | Money market funds
Assets
Total assets measured at estimated fair value	38,685	61,971
Recurring basis | Quoted Price in Active Markets for Identical Assets, Level 1
Assets
Total assets measured at estimated fair value	38,685	61,971
Recurring basis | Quoted Price in Active Markets for Identical Assets, Level 1 | U.S. government securities
Assets
Total assets measured at estimated fair value		0
Recurring basis | Quoted Price in Active Markets for Identical Assets, Level 1 | U.S. government agencies
Assets
Total assets measured at estimated fair value		0
Recurring basis | Quoted Price in Active Markets for Identical Assets, Level 1 | U.S. corporate notes
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Quoted Price in Active Markets for Identical Assets, Level 1 | U.S. commercial papers
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Quoted Price in Active Markets for Identical Assets, Level 1 | Money market funds
Assets
Total assets measured at estimated fair value	38,685	61,971
Recurring basis | Significant Other Observable Inputs, Level 2
Assets
Total assets measured at estimated fair value	11,861	60,738
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government securities
Assets
Total assets measured at estimated fair value		9,942
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government agencies
Assets
Total assets measured at estimated fair value		9,985
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. corporate notes
Assets
Total assets measured at estimated fair value	3,873	10,866
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. commercial papers
Assets
Total assets measured at estimated fair value	7,988	29,945
Recurring basis | Significant Other Observable Inputs, Level 2 | Money market funds
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. government securities
Assets
Total assets measured at estimated fair value		0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. government agencies
Assets
Total assets measured at estimated fair value		0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. corporate notes
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. commercial papers
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | Money market funds
Assets
Total assets measured at estimated fair value	0	0
Nonrecurring basis | Debt securities
Debt
Term B Loan	13,750	243,750
2023 Notes	247,310	241,259
2025 Notes	211,917	205,975
Total fair value of debt	472,977	690,984
Nonrecurring basis | Quoted Price in Active Markets for Identical Assets, Level 1 | Debt securities
Debt
Term B Loan	0	0
2023 Notes	0	0
2025 Notes	0	0
Total fair value of debt	0	0
Nonrecurring basis | Significant Other Observable Inputs, Level 2 | Debt securities
Debt
Term B Loan	13,750	243,750
2023 Notes	247,310	241,259
2025 Notes	211,917	205,975
Total fair value of debt	472,977	690,984
Nonrecurring basis | Significant Unobservable Inputs, Level 3 | Debt securities
Debt
Term B Loan	0	0
2023 Notes	0	0
2025 Notes	0	0
Total fair value of debt	$ 0	$ 0