Collaboration Agreements (Schedule of Collaboration Agreements) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Jun. 30, 2015	Mar. 31, 2016	Mar. 31, 2015
bluebird
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees		$ 10.0	$ 0.0
Milestones		0.0	0.0
Extension/Termination of Agreements		0.0	0.0
Amortization of Prepaid Research and Development		2.1	0.0
Equity Investments Made During Period		0.0	0.0
Juno
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees	$ 150.0	0.0	0.0
Milestones		0.0	0.0
Extension/Termination of Agreements		0.0	0.0
Amortization of Prepaid Research and Development		0.0	0.0
Equity Investments Made During Period		41.0	0.0
Equity Investment Premium Paid		6.0
Other Collaboration Arrangements
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront Fees		70.0	19.0
Milestones		65.0	0.0
Extension/Termination of Agreements		0.0	8.1
Amortization of Prepaid Research and Development		3.9	5.5
Equity Investments Made During Period		$ 37.0	$ 0.0