Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended	12 Months Ended		12 Months Ended							1 Months Ended	12 Months Ended
	Sep. 21, 2012	Mar. 31, 2013	Dec. 31, 2013 Y Segment	Dec. 31, 2012	Dec. 31, 2013 Astellas [Member] Collaborative agreement [Member]	Dec. 31, 2013 Pfizer [Member] Collaborative agreement [Member]	Dec. 31, 2013 Cash and cash equivalents [Member]	Dec. 31, 2013 Short-Term Investments [Member]	Dec. 31, 2012 Capital stock [Member]	Sep. 21, 2012 Capital stock [Member]	Sep. 20, 2012 Capital stock [Member]	Sep. 21, 2012 Common stock [Member]	Dec. 31, 2013 Common stock [Member]	Dec. 31, 2012 Common stock [Member]	Sep. 20, 2012 Common stock [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of operating business segment			1
Shares of capital stock authorized pre-split									51,000,000		86,000,000
Shares of common stock authorized pre-split														50,000,000	85,000,000
Shares of capital stock authorized										171,000,000
Total number of authorized shares			170,000,000	170,000,000								170,000,000
Forward stock split of common stock			2									2	2
Common stock issued in forward stock split	37,000,000
Par value of common stock			$ 0.01	$ 0.01
Debt instrument convertible increment of principal amount of conversion	$ 1,000		$ 1,000
Conversion rate of common stock, shares per principal amount	19.5172
Conversion price, per share of common stock	$ 51.24
Highly liquid investments maximum maturity period							3 months	12 months
Contract term of the Convertible Notes, years			5
Nonrefundable upfront payment received under collaboration arrangement					110,000,000	225,000,000
Amount of net loss increased due to out-of-period adjustment		$ 3,600,000
Out of period adjustment		In the first quarter of 2013, the Company recorded an out-of-period correcting adjustment that increased operating expenses and net loss by $3.6 million for the three months ended March 31, 2013. Management concluded that the adjustment is not material to the full year 2013 results or any previously reported financial statements.