Collaboration Agreements (Narrative) (Details) shares in Millions, $ in Millions	1 Months Ended				3 Months Ended		6 Months Ended
	Apr. 30, 2016 USD ($)	Feb. 29, 2016 USD ($)	Jun. 30, 2015 USD ($) shares	Apr. 30, 2015 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($) shares	Jun. 30, 2016 USD ($) extensions	Jun. 30, 2015 USD ($) shares
Agios Pharmaceuticals Inc
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payment							$ 120.0
Upfront fees					$ 200.0	$ 9.0	200.0	$ 9.0
Milestones					0.0	0.0	$ 25.0	0.0
Initial term of agreement							4 years
Contract extensions | extensions							2
Additional agreement term							1 year
Additional years for contract extension							4 years
Agios Pharmaceuticals Inc | AG-881
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront fees				$ 10.0
Contingent consideration				70.0
Agios Pharmaceuticals Inc | 2016 Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront fees							$ 200.0
Option exercise fee (up to)							30.0
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	$ 50.0		$ 150.0		50.0	0.0	50.0	0.0
Milestones					0.0	$ 0.0	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 | shares			9.1			9.1		9.1
Ownership percentage			9.00%			9.00%		9.00%
Available-for-sale securities in Juno			$ 424.9			$ 424.9		$ 424.9
Initial term of agreement			10 years
Other Collaboration Arrangements
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront fees					34.0	41.0	104.0	60.0
Milestones					$ 10.5	$ 8.0	50.5	$ 8.0
Research and Development | Agios Pharmaceuticals Inc | AG-881
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront fees				9.0
Research and Development | Juno Therapeutics
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront fees			$ 425.1
Manufacturing Rights | Agios Pharmaceuticals Inc | AG-881
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront fees				$ 1.0
Clinical and Regulatory | 2016 Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent consideration							169.0
One Program Designated by Celgene | 2016 Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent consideration							$ 209.0
One Program Designated by Celgene | Agios Pharmaceuticals Inc | 2016 Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Cost and profit share rate							35.00%
Clinical Regulatory and Commercial Milestones | Agios Pharmaceuticals Inc | 2016 Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent consideration							$ 386.0
Celgene
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Allocation of cost, as a percentage			70.00%
Celgene | One Program Designated by Celgene | 2016 Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Cost and profit share rate							65.00%