Marketable securities - Summary of Available for Sale Securities Held (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 174,291
Unrealized Gains	3
Unrealized Losses	(77)
Fair Value	174,217
U.S. Government Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	161,652
Unrealized Gains	3
Unrealized Losses	(48)
Fair Value	161,607
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	12,639
Unrealized Losses	(29)
Fair Value	$ 12,610

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b)

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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-Topic 210

-SubTopic 10

-Section S99

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