Cash, Cash Equivalents, and Short-Term Investments - Summary of Cash, Cash Equivalents, and Short-Term Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Cash And Cash Equivalents And Short Term Investments [Line Items]
Total cash, cash equivalents, and short-term investments, Amortized Cost	$ 497,541
Total cash, cash equivalents, and short-term investments, Gross Unrealized Gains	5
Total cash, cash equivalents, and short-term investments, Gross Unrealized Losses	(39)
Total cash, cash equivalents, and short-term investments, Fair Value	497,507
Cash and cash equivalents [Member]
Cash And Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	421,150	$ 502,677
Cash and cash equivalents, Fair Value	421,150	502,677
Cash and cash equivalents [Member] | Cash [Member]
Cash And Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	224,412	313,646
Cash and cash equivalents, Fair Value	224,412	313,646
Cash and cash equivalents [Member] | Money market funds [Member]
Cash And Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	189,041	189,031
Cash and cash equivalents, Fair Value	189,041	$ 189,031
Cash and cash equivalents [Member] | Commercial paper [Member]
Cash And Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	7,697
Cash and cash equivalents, Fair Value	7,697
Short-term investments [Member]
Cash And Cash Equivalents And Short Term Investments [Line Items]
Amortized Cost	76,391
Gross Unrealized Gains	5
Gross Unrealized Losses	(39)
Fair Value	76,357
Short-term investments [Member] | Commercial paper [Member]
Cash And Cash Equivalents And Short Term Investments [Line Items]
Amortized Cost	21,568
Fair Value	21,568
Short-term investments [Member] | Corporate debt securities [Member]
Cash And Cash Equivalents And Short Term Investments [Line Items]
Amortized Cost	42,830
Gross Unrealized Gains	1
Gross Unrealized Losses	(39)
Fair Value	42,792
Short-term investments [Member] | Federal agency securities [Member]
Cash And Cash Equivalents And Short Term Investments [Line Items]
Amortized Cost	9,993
Gross Unrealized Gains	4
Fair Value	9,997
Short-term investments [Member] | Certificates of deposit [Member]
Cash And Cash Equivalents And Short Term Investments [Line Items]
Amortized Cost	2,000
Fair Value	$ 2,000