Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term fixed income securities	$ 376,087,000
Total assets	534,378,000
Liabilities measured at fair value on a recurring basis	$ 80,707,000	$ 77,412,000
Shares of common stock exercisable through warrants	28,125
Warrants exercise price per share (usd per share)	$ 72.00
Fair value of contingent consideration	$ 32,500,000	0
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	97,686,000
Liabilities measured at fair value on a recurring basis	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	436,692,000
Liabilities measured at fair value on a recurring basis	0	0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	0
Liabilities measured at fair value on a recurring basis	80,707,000	77,412,000
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	607,000	512,000
Common stock warrant liabilities | 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 | 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 | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets			$ 449,000
Liabilities measured at fair value on a recurring basis	607,000	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	80,100,000	76,900,000
Contingent consideration liabilities | 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 | 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 | 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
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	58,127,000
Short-term fixed income securities	174,769,000
Commercial paper | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	0
Short-term fixed income securities	0
Commercial paper | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	58,127,000
Short-term fixed income securities	174,769,000
Commercial paper | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	0
Short-term fixed income securities	0
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	1,624,000
Short-term fixed income securities	47,386,000
Corporate debt securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	0
Short-term fixed income securities	0
Corporate debt securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	1,624,000
Short-term fixed income securities	47,386,000
Corporate debt securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	0
Short-term fixed income securities	0
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	97,686,000	289,782,000
Money market funds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	97,686,000	289,782,000
Money market funds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	0	0
Money market funds | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	0	$ 0
Certificate of deposits
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	854,000
Short-term fixed income securities	118,775,000
Certificate of deposits | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	0
Short-term fixed income securities	0
Certificate of deposits | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	854,000
Short-term fixed income securities	118,775,000
Certificate of deposits | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents	0
Short-term fixed income securities	0
U.S. Treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term fixed income securities	35,157,000
U.S. Treasuries | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term fixed income securities	0
U.S. Treasuries | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term fixed income securities	35,157,000
U.S. Treasuries | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term fixed income securities	0
ZX008
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Potential contingent consideration payment, minimum	0
Potential contingent consideration payment, maximum	95,000,000.0
Fair value of contingent consideration	32,500,000
Total contingent consideration	$ 80,100,000