v3.8.0.1
Fair Value Measurements (Details) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities $ 107,100,000 $ 29,200,000
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount 0 0
Cash Equivalents, at Carrying Value 344,600,000 32,300,000
Available-for-sale Securities, Debt Securities, Current 107,100,000 29,200,000
Leve 3 transfers in or out 0 0
Fair Value, Inputs, Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash Equivalents, at Carrying Value 306,600,000 0
Available-for-sale Securities, Debt Securities, Current 0 0
Fair Value, Inputs, Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash Equivalents, at Carrying Value 38,000,000 32,300,000
Available-for-sale Securities, Debt Securities, Current 107,100,000 29,200,000
Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash Equivalents, at Carrying Value 0 0
Available-for-sale Securities, Debt Securities, Current 0 0
U.S. government agencies    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities 87,300,000 22,200,000
Available-for-sale Securities, Debt Securities, Current 87,300,000 22,200,000
U.S. government agencies | Fair Value, Inputs, Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Debt Securities, Current 0 0
U.S. government agencies | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Debt Securities, Current 87,300,000 22,200,000
U.S. government agencies | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Debt Securities, Current 0 0
Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities 14,700,000 3,200,000
Available-for-sale Securities, Debt Securities, Current 14,700,000 3,200,000
Commercial paper | Fair Value, Inputs, Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Debt Securities, Current 0 0
Commercial paper | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Debt Securities, Current 14,700,000 3,200,000
Commercial paper | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Debt Securities, Current 0 0
Corporate debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities 5,100,000 3,800,000
Available-for-sale Securities, Debt Securities, Current 5,100,000 3,800,000
Corporate debt | Fair Value, Inputs, Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Debt Securities, Current 0 0
Corporate debt | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Debt Securities, Current 5,100,000 3,800,000
Corporate debt | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Debt Securities, Current $ 0 $ 0