Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Marketable Securities - Schedule of Investments (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Schedule Of Available For Sale Securities [Line Items]
Cash and cash equivalents	$ 25,256	$ 6,030	$ 126,823	$ 10,186
Total cash equivalents	17,089	3,182
Amortized Cost	217,533	223,399
Gross Unrealized Gains	32	33
Gross Unrealized (Losses)	(7)	(58)
Estimated Fair Value	217,558us-gaap_AvailableForSaleSecurities	223,374us-gaap_AvailableForSaleSecurities
Available-for-sale securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	200,444	220,217
Gross Unrealized Gains	32	33
Gross Unrealized (Losses)	(7)	(58)
Estimated Fair Value	200,469	220,192
Money market funds [Member]
Schedule Of Available For Sale Securities [Line Items]
Cash and cash equivalents	17,089	3,182
Total cash equivalents	17,089	3,182
Cash equivalents [Member]
Schedule Of Available For Sale Securities [Line Items]
Cash and cash equivalents	17,089	3,182
Total cash equivalents	17,089	3,182
Commercial paper securities [Member] \| Available-for-sale securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	31,121	33,715
Gross Unrealized Gains	27	33
Estimated Fair Value	31,148	33,748
Corporate debt securities [Member] \| Available-for-sale securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	23,261	22,831
Gross Unrealized Gains	5
Gross Unrealized (Losses)		(18)
Estimated Fair Value	23,266	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	146,062	163,671
Gross Unrealized (Losses)	(7)	(40)
Estimated Fair Value	$ 146,055	$ 163,631