Available-for-Sale Securities and Fair Value Measurements - Fair Value Measurements (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Recurring basis
Assets
Total assets measured at estimated fair value	$ 228,292	$ 108,974
Recurring basis | U.S. government securities
Assets
Total assets measured at estimated fair value	49,037	29,733
Recurring basis | U.S. government agencies
Assets
Total assets measured at estimated fair value	19,927	4,971
Recurring basis | U.S. corporate notes
Assets
Total assets measured at estimated fair value		2,875
Recurring basis | U.S. commercial paper
Assets
Total assets measured at estimated fair value	38,747	22,037
Recurring basis | Money market funds
Assets
Total assets measured at estimated fair value	120,581	49,358
Recurring basis | Quoted Price in Active Markets for Identical Assets, Level 1
Assets
Total assets measured at estimated fair value	120,581	49,358
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	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	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
Recurring basis | Quoted Price in Active Markets for Identical Assets, Level 1 | U.S. commercial paper
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	120,581	49,358
Recurring basis | Significant Other Observable Inputs, Level 2
Assets
Total assets measured at estimated fair value	107,711	59,616
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government securities
Assets
Total assets measured at estimated fair value	49,037	29,733
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government agencies
Assets
Total assets measured at estimated fair value	19,927	4,971
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. corporate notes
Assets
Total assets measured at estimated fair value		2,875
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. commercial paper
Assets
Total assets measured at estimated fair value	38,747	22,037
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	0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. government agencies
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. corporate notes
Assets
Total assets measured at estimated fair value		0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. commercial paper
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
Debt
Term B Loan	13,750	13,750
2023 Notes	252,754	258,918
2025 Notes	208,181	230,692
Total fair value of liabilities	474,685	503,360
Nonrecurring basis | Quoted Price in Active Markets for Identical Assets, Level 1 | Debt
Debt
Term B Loan	0	0
2023 Notes	0	0
2025 Notes	0	0
Total fair value of liabilities	0	0
Nonrecurring basis | Significant Other Observable Inputs, Level 2 | Debt
Debt
Term B Loan	13,750	13,750
2023 Notes	252,754	258,918
2025 Notes	208,181	230,692
Total fair value of liabilities	474,685	503,360
Nonrecurring basis | Significant Unobservable Inputs, Level 3 | Debt
Debt
Term B Loan	0	0
2023 Notes	0	0
2025 Notes	0	0
Total fair value of liabilities	$ 0	$ 0