Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	9 Months Ended
	Mar. 31, 2019	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term fixed income securities		$ 221,865	$ 445,733
Total assets		231,396	508,965
Common stock warrant liabilities		102	343
Total liabilities		$ 71,002	78,543
Shares of common stock exercisable through warrants		28,125
Warrants exercise price per share (usd per share)		$ 72.00
Current portion of contingent consideration		$ 35,200	32,300
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		8,535	63,232
Total liabilities		0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		222,861	445,733
Total liabilities		0	0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		0	0
Total liabilities		71,002	78,543
Common stock warrant liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Common stock warrant liabilities		102	343
Common stock warrant liabilities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Common stock warrant liabilities		0	0
Common stock warrant liabilities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Common stock warrant liabilities		0	0
Common stock warrant liabilities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Common stock warrant liabilities		102	343
Contingent consideration liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration liabilities		70,900	78,200
Total contingent consideration		70,900	78,200
Contingent consideration liabilities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration liabilities		0	0
Total contingent consideration		0	0
Contingent consideration liabilities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration liabilities		0	0
Total contingent consideration		0	0
Contingent consideration liabilities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration liabilities		70,900	78,200
Total contingent consideration		70,900	78,200
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents		8,535	63,232
Money market funds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents		8,535	63,232
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
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents		996
Short-term fixed income securities		86,936	152,940
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	0
Commercial paper | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents		996
Short-term fixed income securities		86,936	152,940
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	0
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term fixed income securities		82,257	60,605
Corporate debt securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term fixed income securities		0	0
Corporate debt securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term fixed income securities		82,257	60,605
Corporate debt securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term fixed income securities		0	0
Certificate of deposits
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term fixed income securities		52,672	128,647
Certificate of deposits | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term fixed income securities		0	0
Certificate of deposits | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term fixed income securities		52,672	128,647
Certificate of deposits | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term fixed income securities		0	0
U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term fixed income securities			103,541
U.S. Treasury securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term fixed income securities			0
U.S. Treasury securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term fixed income securities			103,541
U.S. Treasury securities | 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]
Contingent consideration liabilities		70,900
Potential contingent consideration payment, maximum				$ 95,000
Contingent consideration paid	$ 10,000
Total contingent consideration		70,900
Current portion of contingent consideration		$ 35,200