Collaborative Arrangements - Cystic Fibrosis Foundation Therapeutics Incorporated (Details) - USD ($)	1 Months Ended	3 Months Ended		12 Months Ended
	Oct. 31, 2016	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2017
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional milestone payments				$ 0
Cystic Fibrosis Foundation Therapeutics Incorporated
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments		$ 13,900,000	$ 13,900,000	$ 0
Collaborative funding	$ 75,000,000.0
Additional collaborative funding	$ 6,000,000.0