Collaboration Agreements (Narrative) (Details) - USD ($) shares in Millions, $ in Millions	1 Months Ended			3 Months Ended
	Apr. 30, 2016	Feb. 29, 2016	Jun. 30, 2015	Mar. 31, 2016	Mar. 31, 2015
Bluebird bio, Inc.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees			$ 25.0
Contingent consideration			$ 230.0
Milestones		$ 10.0
Termination agreement, period			90 days
Juno Therapeutics
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees			$ 150.0	$ 0.0	$ 0.0
Milestones				$ 0.0	$ 0.0
Termination agreement, period			120 days
Allocation of cost, as a percentage			30.00%
Payments made to Juno			$ 1,000.0
Investment in Juno, shares owned			9.1
Ownership percentage			9.00%
Available-for-sale securities in Juno			$ 424.9
Initial term of agreement			10 years
Research and Development | Juno Therapeutics
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees			$ 425.1
Celgene
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Allocation of cost, as a percentage			70.00%
Subsequent Event | Juno Therapeutics
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees	$ 50.0