Collaboration Agreements (Narrative) (Details) shares in Millions				1 Months Ended						3 Months Ended		9 Months Ended
	Mar. 26, 2015 USD ($)	Mar. 21, 2014 USD ($) Collaborative_Arrangement	Mar. 21, 2014	Sep. 30, 2015 USD ($)	Jul. 31, 2015 USD ($) option_target	Jun. 30, 2015 USD ($) shares	Apr. 30, 2015 USD ($)	Feb. 28, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)
bluebird
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees												$ 9,000,000	$ 0
Milestone payments made												0	0
Payment to terminate a collaboration agreement												0	0
bluebird | Discovery and Development Collaboration and License Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestones payment									$ 20,000,000.0			120,000,000
bluebird | AG-881
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestones payment							$ 70,000,000
Milestone payment, payable							10,000,000
bluebird | AG-881 | Research and Development Collaboration Expense
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payment, payable							9,000,000.0
Epizyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees										$ 0	$ 0	0	0
Milestone payments made										0	0	0	0
Payment to terminate a collaboration agreement										10,000,000	0	10,000,000	0
License payments FORMA
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestones payment												$ 430,000,000.0
FORMA
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, term			3 years 6 months
Research and development period												4 years
Upfront Fees												$ 39,000,000	225,000,000
Milestone payments made												0	0
Payment to terminate a collaboration agreement												0	0
FORMA | Research and development payments
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestones payment												200,000,000
FORMA | Regulatory approval
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestones payment												315,000,000.0
FORMA | First Collaboration Agreement, First Ex-US License
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees								$ 19,000,000.0
FORMA | Second Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees		$ 225,000,000.0
Term of additional agreement		2 years
Payment to exercise additional agreements		$ 375,000,000
FORMA | Second Collaboration Agreement | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of additional agreement | Collaborative_Arrangement		2
FORMA | Second Collaboration Agreement, First Ex-US License
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees							20,000,000.0
MorphoSys
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees												0	0
Milestone payments made												0	0
Payment to terminate a collaboration agreement	$ 8,100,000											8,100,000	0
Other Collaboration Arrangements
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees										26,900,000	6,000,000	47,900,000	54,000,000
Milestone payments made										0	6,800,000	8,000,000	7,300,000
Payment to terminate a collaboration agreement										$ 0	$ 0	0	0
Acceleron
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees												0	0
Milestone payments made												0	0
Payment to terminate a collaboration agreement												0	0
NantBioScience
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees												0	50,000,000
Milestone payments made												0	0
Payment to terminate a collaboration agreement												0	0
NantBioScience | Selling, general and administrative
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees													$ 25,000,000
Collaboration and License Agreement | Epizyme | HMT Inhibitor Targeting Option Targets
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payment, payable					$ 10,000,000.0
Option targets | option_target					3
Exclusive licenses | option_target					2
Development milestone payment					$ 75,000,000.0
Regulatory milestone payment					365,000,000.0
Sales milestone payment					170,000,000.0
Collaboration and License Agreement | Epizyme | DOT1L Program
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Regulatory milestone payment					100,000,000.0
Additional development milestone payments					$ 35,000,000.0
Collaboration and License Agreement | Epizyme | Molecule HMT Inhibitor Targeting DOT1L
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development period					3 years
Termination agreement, period					120 days
Collaboration Agreement | Epizyme | B-Cell Maturation Antigen (BCMA)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contingent consideration						$ 230,000,000.0
Milestone payment, payable						$ 25,000,000
Termination agreement, period						90 days
Co-development agreement						0.50
Collaboration Agreement | Nurix, Inc | Immune Modulator Assets
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payment, payable				$ 149,800,000
Research and Development Arrangement, Contract to Perform for Others, Equity Investment				$ 17,000,000
Research and development arrangement, contract to perform for others, equity interest				13
Research and development arrangement contract to perform for others, period after delivery of phase I data package				45 days
Research and development arrangement, contract to perform for others, future success-based milestone payments				$ 405,000,000
Research and development arrangement, contract to perform for others, agreement term				4 years
Strategic Collaboration Agreement | MedImmune Limited | MEDI4736
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payment, payable							$ 450,000,000
Termination agreement, period							9 years
Research and development arrangement, contract to perform for others, cost sharing percentage, period two							7500.00%
Research and development arrangement, contract to perform for others, royalty rate, percentage of worldwide sales							7000.00%
Research and development arrangement, contract to perform for others, final royalty rate							5000.00%
Research and development arrangement, contract to perform for others, royalty rate, period to reach final rate							4 years
Collaboration and Option Agreement | Sutro | Immune Modulator Assets
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payment, payable						$ 82,500,000
Research and development arrangement, contract to perform for others, future success-based milestone payments						$ 190,000,000.0
Termination agreement, period						6 months
Research and development arrangement, contract to perform for others, agreement term						3 years
Collaboration and Option Agreement | Sutro | Immune Modulator Assets | Research and Development Collaboration Expense
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payment, payable						$ 69,500,000
Collaboration and Option Agreement | Acetylon | Attainment of Developmental, Regulatory and Sales Milestones [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development arrangement, contract to perform for others, future success-based milestone payments												$ 120,000,000.0
Collaboration and Investment Agreement | Juno Therapeutics, Inc | Development and Commercialization of Immuno-Therapeutics
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payment, payable						150,000,000
Research and development arrangement, contract to perform for others, premium paid on equity investment						$ 425,100,000
Termination agreement, period						120 days
Research and development arrangement, contract to perform for others, agreement term						10 years
Research and development arrangement, contract to perform for others, cost and profit sharing percentage, agreement owner						70
Research and development arrangement, contract to perform for others, cost and profit sharing percentage, counterparty						30
Research and development arrangement, investment in counterparty, consideration to be transferred						$ 1,000,000,000
Research and development arrangement, contract to perform for others, shares of counterparty common stock | shares						9.1
Research and development arrangement, contract to perform for others, percentage of counterparty common stock						0.09
Reserach and Development Arrangement Investment in Counterparty, Value of Shares						$ 424,900,000
Manufacturing Rights Asset | bluebird | AG-881
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payment, payable							$ 1,000,000.0
Noncurrent Assets | Collaboration and Option Agreement | Sutro | Immune Modulator Assets
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payment, payable						13,000,000
Equity Investment Asset | Collaboration and Option Agreement | Sutro | Immune Modulator Assets
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payment, payable						10,000,000
Option to license | Collaboration and Option Agreement | Sutro | Immune Modulator Assets
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payment, payable						$ 3,000,000