Subsequent Events (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended
	May 31, 2014 Collaboration Agreement with Avalanche Biotechnologies, inc. [Member]	Mar. 31, 2014 Convertible Debt [Member]	Dec. 31, 2013 Convertible Debt [Member]
Subsequent Event [Line Items]
Debt Conversion, Converted Instrument, Amount		$ 61.1
Debt Instrument, Interest Rate, Stated Percentage		1.875%	1.875%
Trading Days for cash settlement		40
Research and Development Expense	6.0
Research and Development Asset Acquired Other than Through Business Combination, Written-off	2.0
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