Collaborative Arrangement - Cystic Fibrosis Foundation Therapeutics Incorporated (Details) - USD ($)	1 Months Ended	3 Months Ended			12 Months Ended
	Apr. 30, 2011	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2013	Dec. 31, 2012
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative revenues		$ 74,000		$ 842,000
Cystic Fibrosis Foundation Therapeutics Incorporated
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative funding	$ 75,000,000
Number of years over which funding will be made (in years)	5 years
Collaborative revenues		$ 0		$ 0
Milestone Revenues			$ 13,900,000		$ 9,300,000	$ 9,300,000