Available-for-Sale Securities and Fair Value Measurements - Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Recurring basis
Assets
Total assets measured at estimated fair value	$ 162,580	$ 148,813
Recurring basis | U.S. government agencies
Assets
Total assets measured at estimated fair value	9,959	12,429
Recurring basis | U.S. corporate notes
Assets
Total assets measured at estimated fair value	9,876
Recurring basis | U.S. commercial paper
Assets
Total assets measured at estimated fair value	24,582	72,065
Recurring basis | Money market funds
Assets
Total assets measured at estimated fair value	118,163	64,319
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1
Assets
Total assets measured at estimated fair value	118,163	64,319
Recurring basis | Quoted Prices 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 Prices in Active Markets for Identical Assets, Level 1 | U.S. corporate notes
Assets
Total assets measured at estimated fair value	0
Recurring basis | Quoted Prices 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 Prices in Active Markets for Identical Assets, Level 1 | Money market funds
Assets
Total assets measured at estimated fair value	118,163	64,319
Recurring basis | Significant Other Observable Inputs, Level 2
Assets
Total assets measured at estimated fair value	44,417	84,494
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government agencies
Assets
Total assets measured at estimated fair value	9,959	12,429
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. corporate notes
Assets
Total assets measured at estimated fair value	9,876
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. commercial paper
Assets
Total assets measured at estimated fair value	24,582	72,065
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 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
Liabilities
Total fair value of liabilities	686,783	689,314
Nonrecurring basis | Debt Securities
Liabilities
Term B Loan	250,000
2023 Notes	234,658	202,125
2025 Notes	202,125
2029 Notes		487,189
Nonrecurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1
Liabilities
Total fair value of liabilities	0	0
Nonrecurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | Debt Securities
Liabilities
Term B Loan	0
2023 Notes	0	0
2025 Notes	0
2029 Notes		0
Nonrecurring basis | Significant Other Observable Inputs, Level 2
Liabilities
Total fair value of liabilities	686,783	689,314
Nonrecurring basis | Significant Other Observable Inputs, Level 2 | Debt Securities
Liabilities
Term B Loan	250,000
2023 Notes	234,658	202,125
2025 Notes	202,125
2029 Notes		487,189
Nonrecurring basis | Significant Unobservable Inputs, Level 3
Liabilities
Total fair value of liabilities	0	0
Nonrecurring basis | Significant Unobservable Inputs, Level 3 | Debt Securities
Liabilities
Term B Loan	0
2023 Notes	0	0
2025 Notes	$ 0
2029 Notes		$ 0