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Fair Values of Financial Instruments - Additional Information (Detail) (Series A, Senior Notes, 4.97 Percent, Due October 5th 2020) | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2012 | Dec. 31, 2011 | Oct. 05, 2010
Series A, Senior Notes, 4.97 Percent, Due October 5th 2020 |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Debt interest rate | 4.97% | 4.97% | 4.97%
Current market rates used in measuring fair value | 3.91% | 4.07% |

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- Definition Interest rate stated in the contractual debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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