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%

Interest rate stated in the contractual debt agreement.

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

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