Collaboration Agreements (Avalanche Biotechnologies, Inc.) (Details) (USD $)	9 Months Ended		1 Months Ended	7 Months Ended	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Jul. 31, 2014 Collaboration Agreement with Avalanche Biotechnologies, inc.	Jul. 31, 2014 Collaboration Agreement with Avalanche Biotechnologies, inc.	Sep. 30, 2014 Collaboration Agreement with Avalanche Biotechnologies, inc.	Sep. 30, 2014 Per Product Collaboration Agreement with Avalanche Biotechnologies, inc.
Collaboration Agreement [Line Items]
Research and Development Asset Acquired Other than Through Business Combination, Written-off					$ 2,000,000
Pre-payment of collaboration research costs					6,000,000
Potential future milestone payments					640,000,000	80,000,000
Number of therapeutic targets					8
Payment to acquire preferred share of non-public entity					5,000,000
Number of therapeutic targets of Avalanche's choice					2
Payments to Acquire Marketable Securities	$ 478,436,000	$ 477,312,000		$ 10,000,000
Number of shares of Common Stock to be purchased			588,235