Other Expense, Net - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	51 Months Ended
Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013
Component of Other Expense, Nonoperating [Line Items]
Payment for expansion of transmission technologies	$ 2.5
Impairment loss on investments in technology-related initiatives	2.5
Grant Program income	1.2	2.8	42.0
Reduction of basis of capital assets purchased under Grant program	1.4	1.5
Assets in service under the Grant Program	7.1
Grant Program, related depreciation	$ 0.1	$ 0.1

Amount recognized as an operating expense or loss during the period to reduce the carrying amount of a project that has been impaired but not abandoned.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 45

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=8077374&loc=d3e2420-110228

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 144

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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_ImpairmentOfOngoingProject
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration