Collaboration Agreements - Alexion – 2014 Strategic Alliance in Rare Diseases (Details)	1 Months Ended	3 Months Ended								12 Months Ended
	Jan. 31, 2014 USD ($) candidate	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Deferred revenue recognized										$ 51,987,000
Collaboration revenue		$ 14,055,000	$ 17,046,000	$ 13,083,000	$ 16,025,000	$ 35,421,000	$ 41,757,000	$ 28,851,000	$ 29,039,000	60,209,000	$ 135,068,000	$ 205,825,000
Deferred revenue		199,528,000				240,924,000				199,528,000	240,924,000
Collaboration revenue
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue										42,803,000	76,519,000	146,953,000
2014 Alexion Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Upfront cash payments	$ 100,000,000
Proceeds from equity investment	$ 25,000,000
Number of product candidates | candidate	10
Arrangement term	10 years
Deferred revenue recognized											70,300,000
Deferred revenue		$ 0				$ 0				0	$ 0
2014 Alexion Agreements | Collaboration revenue
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue										$ 0		$ 74,400,000