Collaborative Arrangements and Acquisitions - Cystic Fibrosis Foundation Therapeutics Incorporated (Details) - Cystic Fibrosis Foundation Therapeutics Incorporated - USD ($)	1 Months Ended	3 Months Ended	9 Months Ended
	Oct. 31, 2016	Mar. 31, 2016	Sep. 30, 2017
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments		$ 13,900,000
Additional milestone payments			$ 0
Collaborative funding	$ 75,000,000.0
Additional collaborative funding	$ 6,000,000.0