Collaboration Agreements (Avalanche Biotechnologies, Inc.) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Collaboration Agreements [Line Items]
Payments to Acquire Marketable Securities	$ 564,188,000	$ 577,278,000	$ 470,945,000
Collaboration Agreement, Avalanche Biotechnologies, Inc
Collaboration Agreements [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
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	10,000,000
Number of Shares of Common Stock had purchased	588,235
Product | Collaboration Agreement, Avalanche Biotechnologies, Inc
Collaboration Agreements [Line Items]
Potential future milestone payments	$ 80,000,000