Summary of Significant Accounting Policies - Additional Information (Detail)		3 Months Ended			12 Months Ended
	Sep. 15, 2015 $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares	Mar. 31, 2013 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2015 USD ($) Segment ReportingUnit $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares	Dec. 31, 2013 USD ($)	Jun. 30, 2015 USD ($)	Jun. 15, 2015 shares
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Number of operating business segment | Segment					1
Shares of common stock authorized | shares		340,000,000			340,000,000	340,000,000
Stock split of common stock	2
Stock dividend, common stock shares issued | shares					81,700,000
Common stock, par value | $ / shares		$ 0.01			$ 0.01	$ 0.01
Repayment of convertible notes in cash					$ 258,742,000	$ 12,000
Deferred revenue		$ 2,822,000				2,822,000
Realized gains or losses from sales of available-for-sale securities						0	$ 0
Number of reporting unit | ReportingUnit					1
Contract term of the Convertible Notes, years					5 years
Discrete tax benefit		33,400,000
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.
Amount of net loss increased due to out-of-period adjustment			$ 3,600,000
Cash And Cash Equivalents
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Highly liquid investments maximum maturity period					3 months
Short-Term Investments [Member]
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Highly liquid investments maximum maturity period					12 months
Collaborative agreement [Member]
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Deferred revenue		$ 2,800,000			$ 0	$ 2,800,000
Astellas [Member] | Collaborative agreement [Member]
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Nonrefundable upfront payment received under collaboration arrangement				$ 110,000,000
Deferred revenue					$ 0
2.625% Convertible Senior Notes Due 2017 [Member] | Senior Notes [Member]
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Aggregate principal amount of convertible notes settled								$ 258,800,000
Debt instrument interest rate								2.625%
Debt instrument maturity date					Apr. 01, 2017
Repayment of convertible notes in cash					$ 259,900,000
Common stock, shares issued on redemption of convertible notes | shares					5,638,576
Common Stock [Member]
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Stock split of common stock	2				2
Common stock split, description					On September 15, 2015, the Company effected a two-for-one forward stock split of its common stock in the form of a stock dividend. Stockholders of record as of August 13, 2015 received one additional share of the Company’s common stock, par value $0.01, for each share they held as of the record date.
Stock dividend, common stock shares issued | shares	81,700,000
Common stock, par value | $ / shares	$ 0.01
Stock dividend, effective date of stock split					Sep. 15, 2015
Common stock dividend, record date					Aug. 13, 2015
Pre-Amended and Restated Certificate of Incorporation [Member] | Capital stock [Member]
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Shares of capital stock authorized | shares									171,000,000
Pre-Amended and Restated Certificate of Incorporation [Member] | Common Stock [Member]
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Shares of common stock authorized | shares									170,000,000
Post-Amended and Restated Certificate of Incorporation [Member] | Capital stock [Member]
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Shares of capital stock authorized | shares									341,000,000
Post-Amended and Restated Certificate of Incorporation [Member] | Common Stock [Member]
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Shares of common stock authorized | shares									340,000,000