OPEB Plan Assets, Fair Value Measurements by Input Level (Detail) (Other Postretirement Benefit Plans, USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Life Settlement Contracts, Fair Value Method, Face Value	$ 11.5	$ 11.7
Fair Value, Inputs, Level 1 [Member]
Life Settlement Contracts, Fair Value Method, Face Value	0	0
Fair Value, Inputs, Level 2 [Member]
Life Settlement Contracts, Fair Value Method, Face Value	0	0
Fair Value, Inputs, Level 3 [Member]
Life Settlement Contracts, Fair Value Method, Face Value	$ 11.5	$ 11.7

Face value (death benefits) of the life insurance policies underlying the life settlement contracts accounted for under the fair value method.

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Life Settlement Contract

-URI http://asc.fasb.org/extlink&oid=6516907

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 325

-SubTopic 30

-Section 50

-Paragraph 8

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6385267&loc=d3e42732-111610

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FTB85-4-1

-Paragraph 17

-Subparagraph c

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

Name:	us-gaap_LifeSettlementContractsFairValueMethodFaceValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant