Financial instruments - Quantitative Information about Level 3 Fair Value Measurements (Detail) (Level 3 [Member], Auction Rate Securities [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Principal	$ 115,308
Valuation Adjustment	7,744
Fair Value	$ 107,564
Valuation Technique	Discounted Cash Flow
Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	0.80%
Duration	5 years
Current Yield	0.18%
Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount Rate	2.57%
Duration	8 years
Current Yield	1.34%

Fair value of the level 3 assets used for the quantitative disclosure.

No definition available.

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- Definition

Fair value unobservable input current yield.

+ References

No definition available.

+ Details

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- Definition

Fair value unobservable inputs duration.

+ References

No definition available.

+ Details

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- Definition

Amount represents the principal amount of the level 3 assets used for the quantitative disclosure.

+ References

No definition available.

+ Details

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- Definition

Valuation adjustment for the level 3 assets used for quantitative disclosure.

+ References

No definition available.

+ Details

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- Details

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- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (bbb)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

+ Details

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- Definition

Description of the inputs and valuation technique(s) used to measure fair value.

+ References

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