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

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Marketable Securities - Summary of Cash Equivalents and Available-for-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Schedule Of Available For Sale Securities [Line Items]
Cash and cash equivalents	$ 6,030	$ 10,186	$ 21,679	$ 16,766
Total cash equivalents	3,182	6,934
Amortized Cost	223,399	128,212
Gross Unrealized Gains	33	12
Gross Unrealized (Losses)	(58)
Estimated Fair Value	223,374us-gaap_AvailableForSaleSecurities	128,224us-gaap_AvailableForSaleSecurities
Cash equivalents [Member]
Schedule Of Available For Sale Securities [Line Items]
Cash and cash equivalents	3,182	6,934
Total cash equivalents	3,182	6,934
Available-for-sale securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	220,217
Gross Unrealized Gains	33
Gross Unrealized (Losses)	(58)
Estimated Fair Value	220,192
Money market funds [Member] \| Cash equivalents [Member]
Schedule Of Available For Sale Securities [Line Items]
Cash and cash equivalents	3,182	6,934
Total cash equivalents	3,182	6,934
Commercial paper securities [Member] \| Available-for-sale securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	33,715
Gross Unrealized Gains	33
Estimated Fair Value	33,748
Corporate debt securities [Member] \| Available-for-sale securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	22,831
Gross Unrealized (Losses)	(18)
Estimated Fair Value	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	163,671	121,278
Gross Unrealized Gains		12
Gross Unrealized (Losses)	(40)
Estimated Fair Value	$ 163,631	$ 121,290