Collaboration Agreements (Schedule of Collaboration Agreements) (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended			6 Months Ended
	Apr. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Agios Pharmaceuticals Inc
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees			$ 200.0		$ 9.0	$ 200.0	$ 9.0
Milestones			0.0		0.0	25.0	0.0
Extension/Termination of Agreements			0.0		0.0	0.0	0.0
Amortization of Prepaid Research and Development			0.2		0.0	0.2	0.0
Equity Investments Made During Period			0.0		0.0	0.0	0.0
AstraZeneca
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees			0.0		450.0	0.0	450.0
Milestones			0.0		0.0	0.0	0.0
Extension/Termination of Agreements			0.0		0.0	0.0	0.0
Amortization of Prepaid Research and Development			0.0		0.0	0.0	0.0
Equity Investments Made During Period			0.0		0.0	0.0	0.0
bluebird
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees			0.0		0.0	10.0	0.0
Milestones			0.0		0.0	0.0	0.0
Extension/Termination of Agreements			0.0		0.0	0.0	0.0
Amortization of Prepaid Research and Development			2.1		0.7	4.2	0.7
Equity Investments Made During Period			0.0		0.0	0.0	0.0
Juno
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
Extension/Termination of Agreements			0.0		0.0	0.0	0.0
Amortization of Prepaid Research and Development			0.0		0.0	0.0	0.0
Equity Investments Made During Period			0.0		0.0	41.0	0.0
Equity Investment Premium Paid				$ 6.0
Lycera
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees			0.0		69.5	0.0	69.5
Milestones			0.0		0.0	0.0	0.0
Extension/Termination of Agreements			0.0		0.0	0.0	0.0
Amortization of Prepaid Research and Development			0.0		0.0	0.0	0.0
Equity Investments Made During Period			0.0		10.0	0.0	10.0
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
Extension/Termination of Agreements			0.0		0.0	0.0	8.1
Amortization of Prepaid Research and Development			1.5		6.6	5.4	12.1
Equity Investments Made During Period			$ 0.0		$ 50.0	$ 37.0	$ 50.0