Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	0 Months Ended	3 Months Ended		12 Months Ended
	Sep. 21, 2012	Dec. 31, 2014	Mar. 31, 2013	Dec. 31, 2014 Segment Reporting_Unit	Dec. 31, 2013	Dec. 31, 2012	Sep. 20, 2012
Number of operating business segment				1
Total number of authorized shares		170,000,000		170,000,000	170,000,000
Common stock issued in forward stock split	37,000,000
Par value of common stock		$ 0.01		$ 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			19.5172
Conversion price, per share of common stock	$ 51.24	$ 51.24		$ 51.24
Contract term of the Convertible Notes, years				5 years
Number of reporting unit				1
Discrete tax benefit		33,400,000
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.
Astellas [Member] | Collaborative agreement [Member]
Nonrefundable upfront payment received under collaboration arrangement				110,000,000
Pfizer [Member] | Collaborative agreement [Member]
Nonrefundable upfront payment received under collaboration arrangement				$ 225,000,000
Cash and cash equivalents [Member]
Highly liquid investments maximum maturity period				3 months
Short-Term Investments [Member]
Highly liquid investments maximum maturity period				12 months
Capital stock [Member]
Shares of capital stock authorized	171,000,000
Capital stock [Member] | Pre-Amended and Restated Certificate of Incorporation [Member]
Shares of capital stock authorized pre-split						51,000,000
Capital stock [Member] | After-Amended and Restated Certificate of Incorporation [Member]
Shares of capital stock authorized pre-split							86,000,000
Common Stock [Member]
Total number of authorized shares	170,000,000
Forward stock split of common stock	2			2
Common Stock [Member] | Pre-Amended and Restated Certificate of Incorporation [Member]
Shares of common stock authorized pre-split						50,000,000
Common Stock [Member] | After-Amended and Restated Certificate of Incorporation [Member]
Shares of common stock authorized pre-split							85,000,000