Marketable securities - Summary of Available for Sale Securities Held (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 336,263	$ 144,229
Unrealized Gains	22	6
Unrealized Losses	(145)	(77)
Fair Value	336,140us-gaap_AvailableForSaleSecurities	144,158us-gaap_AvailableForSaleSecurities
U.S. Government Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	323,863	131,589
Unrealized Gains	19	6
Unrealized Losses	(141)	(59)
Fair Value	323,741	131,536
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	12,400	12,640
Unrealized Gains	3
Unrealized Losses	(4)	(18)
Fair Value	$ 12,399	$ 12,622

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

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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