Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($)	6 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shares of common stock exercisable through warrants	28,125
Warrants exercise price per share (usd per share)	$ 72.00
Fair value of contingent consideration	$ 18,500,000		$ 0
Money market fund shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on a recurring basis	252,894,000		289,782,000
Money market fund shares | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on a recurring basis	252,894,000		289,782,000
Money market fund shares | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on a recurring basis	0		0
Money market fund shares | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value on a recurring basis	0		0
Common Stock Warrant Liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value on a recurring basis	543,000		512,000
Common Stock Warrant Liabilities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value on a recurring basis	0		0
Common Stock Warrant Liabilities | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value on a recurring basis	0		0
Common Stock Warrant Liabilities | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value on a recurring basis	543,000		512,000
Contingent Consideration Liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value on a recurring basis	74,400,000		76,900,000
Contingent Consideration Liabilities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value on a recurring basis	0		0
Contingent Consideration Liabilities | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value on a recurring basis	0		0
Contingent Consideration Liabilities | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value on a recurring basis			$ 76,900,000
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	74,400,000	$ 76,900,000		$ 53,900,000	$ 53,400,000	$ 52,800,000
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Potential contingent consideration payment, minimum	0
Potential contingent consideration payment, maximum	95,000,000
Fair value of contingent consideration	$ 18,500,000