Fair Value of Financial Instruments - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Jul. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Jun. 30, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible notes, fair value		$ 198,000,000
PDL BioPharma
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Proceeds from collaborators	$ 50,000,000	1,400,000
PDL BioPharma | Royalty Financing Agreement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of royalty financing		48,700,000
Unamortized issuance costs		4,300,000
PDL BioPharma | Royalty Financing Agreement | Level 3 Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of the carrying value of royalty financing		45,200,000
Regenxbio Inc
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accumulated other comprehensive income, before tax		11,400,000
Available for sale securities carrying value		$ 0
Regenxbio Inc | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities consist shares of common stock		687,139
Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible notes, face value		$ 200,000,000	$ 200,000,000	$ 200,000,000