Quarterly Financial Data - Additional Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
Dec. 31, 2011
Effect of Fourth Quarter Events [Line Items]
Pension and OPEB credits, increase in earnings per share	$ 0.27
Before tax [Member]
Effect of Fourth Quarter Events [Line Items]
Pension and OPEB credits	$ 5.4
After tax [Member]
Effect of Fourth Quarter Events [Line Items]
Pension and OPEB credits	$ 3.3

Amount of the item included in the fourth quarter that affected period-to-period comparability (excluding restatement adjustments), or the extraordinary item or unusual or infrequent item recognized in the fourth quarter in the absence of either a fourth-quarter report or disclosure in the annual report for the fourth quarter.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 270

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=20225539&loc=d3e1570-108306

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 28

-Paragraph 31

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration