Fair Value Measurements - Additional Information (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets (liabilities)	$ 0	$ 0
Fair Value, Measurements, Recurring | U.S. Treasuries | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers of assets from Level 1 to Level 2	149,600,000
Fair Value, Measurements, Recurring | Government securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers of assets from Level 1 to Level 2	$ 42,600,000	$ 11,000,000