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Marketable Securities - Schedule of Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Schedule Of Available For Sale Securities [Line Items]
Cash and cash equivalents	$ 10,768	$ 6,030	$ 99,270	$ 10,186
Total cash equivalents	7,229	3,182
Amortized Cost	214,642	223,399
Gross Unrealized Gains	13	33
Gross Unrealized (Losses)	(16)	(58)
Estimated Fair Value	214,639	223,374
Available-for-sale securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	207,413	220,217
Gross Unrealized Gains	13	33
Gross Unrealized (Losses)	(16)	(58)
Estimated Fair Value	207,410	220,192
Money market funds [Member]
Schedule Of Available For Sale Securities [Line Items]
Cash and cash equivalents	7,229	3,182
Total cash equivalents	7,229	3,182
Cash equivalents [Member]
Schedule Of Available For Sale Securities [Line Items]
Cash and cash equivalents	7,229	3,182
Total cash equivalents	7,229	3,182
Commercial paper securities [Member] \| Available-for-sale securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	25,536	33,715
Gross Unrealized Gains	13	33
Estimated Fair Value	25,549	33,748
Corporate debt securities [Member] \| Available-for-sale securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	28,506	22,831
Gross Unrealized (Losses)	(15)	(18)
Estimated Fair Value	28,491	22,813
U.S. government sponsored entity debt securities [Member] \| Available-for-sale securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	153,371	163,671
Gross Unrealized (Losses)	(1)	(40)
Estimated Fair Value	$ 153,370	$ 163,631