Collaboration Agreements (Avalanche Biotechnologies, Inc.) (Details) (Collaboration Agreement with Avalanche Biotechnologies, inc., USD $) In Millions, except Share data, unless otherwise specified	6 Months Ended		1 Months Ended
	Jun. 30, 2014	Jun. 30, 2014 Per Product	Jul. 31, 2014 Subsequent Event
Collaboration Agreement [Line Items]
Research and Development Asset Acquired Other than Through Business Combination, Written-off	$ 2.0
Pre-payment of collaboration research costs	6.0
Potential future milestone payments	640.0	80.0
Number of therapeutic targets	8
Payment to acquire preferred share of non-public entity	5.0
Number of therapeutic targets of Avalanche's choice	2
Payment to acquire common stock from public entity			$ 10.0
Number of shares of Common Stock to be purchased			588,235