Marketable Debt Securities (Detail) (Truck and Other, USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
AMORTIZED COST	$ 937.9	$ 900.7
UNREALIZED GAINS	10.2	10.0
UNREALIZED LOSSES	0.3	0.6
FAIR VALUE	947.8	910.1
Domestic Nontaxable Debt Securities
Schedule of Available-for-sale Securities [Line Items]
AMORTIZED COST	294.2	291.9
UNREALIZED GAINS	2.6	2.6
UNREALIZED LOSSES	0.1	0.1
FAIR VALUE	296.7	294.4
Domestic Corporate Debt Securities
Schedule of Available-for-sale Securities [Line Items]
AMORTIZED COST	28.9	27.4
UNREALIZED GAINS	0.3	0.3
UNREALIZED LOSSES		0.2
FAIR VALUE	29.2	27.5
US Treasury and Government
Schedule of Available-for-sale Securities [Line Items]
AMORTIZED COST	1.5	1.9
FAIR VALUE	1.5	1.9
Foreign Government Debt Securities
Schedule of Available-for-sale Securities [Line Items]
AMORTIZED COST	384.5	361.2
UNREALIZED GAINS	5.9	6.0
UNREALIZED LOSSES	0.2	0.1
FAIR VALUE	390.2	367.1
Foreign Corporate Debt Securities
Schedule of Available-for-sale Securities [Line Items]
AMORTIZED COST	149.2	148.0
UNREALIZED GAINS	0.9	0.5
UNREALIZED LOSSES		0.2
FAIR VALUE	150.1	148.3
Other Debt Securities
Schedule of Available-for-sale Securities [Line Items]
AMORTIZED COST	79.6	70.3
UNREALIZED GAINS	0.5	0.6
FAIR VALUE	$ 80.1	$ 70.9

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References+ Details

Investments in debt and equity securities which are categorized neither as trading securities nor held-to-maturity securities and which are intended be sold or mature within one year from the balance sheet date or the normal operating cycle, whichever is longer. Such securities are reported at fair value; unrealized gains (losses) related to Available-for-sale Securities are excluded from earnings and reported in a separate component of shareholders' equity (other comprehensive income), unless the Available-for-sale security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding gain (loss) of an Available-for-sale security that is designated as being hedged in a fair value hedge is recognized in earnings during the period of the hedge, as are other than temporary declines in fair value below the cost basis for investments in equity securities and debt securities that an entity intends to sell or it is more likely than not that it will be required to sell before the recovery of its amortized cost basis. Other than temporary declines in fair value below the cost basis for debt securities categorized as Available-for-sale that an entity does not intend to sell and for which it is not more likely than not that the entity will be required to sell before the recovery of its amortized cost basis are bifurcated into credit losses and losses related to all other factors. Other than temporary declines in fair value below cost basis related to credit losses are recognized in earnings, and losses related to all other factors are recognized in other comprehensive income.

+ References

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

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

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Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

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-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.

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

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-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.

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

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Reference 8: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Research Bulletin (ARB)

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+ Details

- Definition

This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

+ References+ Details