Variable Interest Entities - Carrying Amounts of Assets and Liabilities of Consolidated VIE (Detail) (Variable interest entity (VIE) [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Variable interest entity (VIE) [Member]
Variable Interest Entity [Line Items]
Operating assets, net of accumulated depreciation	$ 99.7	$ 104.3
Nonrecourse debt	$ 37.5	$ 45.2

The carrying amount of the consolidated Variable Interest Entity's liabilities included in the reporting entity's statement of financial position.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 810

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (bb)

-URI http://asc.fasb.org/extlink&oid=7880789&loc=d3e5710-111685

Name:	us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant