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Marketable investments - Additional information (Detail) (USD $) In Millions, unless otherwise specified |  |  | 6 Months Ended
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Jun. 30, 2013 Accumulated Net Unrealized Investment Gain (Loss) [Member] | Dec. 31, 2012 Accumulated Net Unrealized Investment Gain (Loss) [Member] | Jun. 30, 2013 ARS [Member] Level 3 [Member] | Jun. 30, 2013 Minimum [Member] | Jun. 30, 2012 Minimum [Member] | Jun. 30, 2013 Maximum [Member] | Jun. 30, 2012 Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Intervals for each security |  |  |  | 7 days |  | 35 days |
Interest rates on securities ranged |  |  |  | 0.10% | 0.10% | 0.40% | 0.50%
Weighted average interest rate |  |  | 1.70% |  |  |  |
Weighted average discount rate |  |  | 5.30% |  |  |  |
Weighted average holding period |  |  | 8 years 8 months 12 days |  |  |  |
Unrealized loss recorded in accumulated other comprehensive income (loss) | $ 2.1 | $ 2.0 |  |  |  |  |

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- Definition Intervals for each security. + References No definition available. + Details
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- Definition The interest rate for investments that have an interest rate. For fixed rate investments, this indicates the fixed interest rate. If the investment has a variable interest rate, the rate stated here may be the rate that is currently relevant. In this case the "Investment, Interest Rate is Current Rate for Variable Rate Investment Flag", "Investment, Interest Rate is Discount Rate at Purchase Flag", "Investment, Interest Rate Reflects Current Yield Flag" should be used. If one of these flags are not appropriate to explain the interest rate that is indicated, then a new flag should be added or an explanation should be included in "Investment, Additional Information". + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 12 -Sentence Column A + Details
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- Definition Accumulated appreciation or loss, net of tax, in value of the total of available-for-sale securities at the end of an accounting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=28358780&loc=SL7669686-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=28358780&loc=d3e637-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=28358780&loc=d3e681-108580 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=28358780&loc=SL7669646-108580 + Details
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