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Summary of Effects on the Consolidated Statement of Comprehensive Income by Retrospective Application of ASU 2017-07 (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Income Before Income Taxes	$ 668.8	$ 581.8	$ 2,057.8	$ 1,569.0
Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues	4,653.6		13,836.4
Income Before Income Taxes	$ 573.6		$ 1,798.1
ASU 2017-07 \| Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues		4,055.6		11,210.5
Selling, general and administrative		116.5		342.8
Interest and other (income), net		(9.8)		(35.7)
Income Before Income Taxes		502.2		1,353.4
ASU 2017-07 \| Truck, Parts and Other \| Previously Reported
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues		4,046.8		11,184.2
Selling, general and administrative		112.9		332.0
Interest and other (income), net		3.2		3.1
Income Before Income Taxes		501.6		1,351.7
ASU 2017-07 \| Truck, Parts and Other \| Effect of Change
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues		8.8		26.3
Selling, general and administrative		3.6		10.8
Interest and other (income), net		(13.0)		(38.8)
Income Before Income Taxes		0.6		1.7
ASU 2017-07 \| Financial Services
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Selling, general and administrative		28.4		81.0
Income Before Income Taxes		70.6		189.8
ASU 2017-07 \| Financial Services \| Previously Reported
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Selling, general and administrative		27.8		79.3
Income Before Income Taxes		71.2		191.5
ASU 2017-07 \| Financial Services \| Effect of Change
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Selling, general and administrative		0.6		1.7
Income Before Income Taxes		$ (0.6)		$ (1.7)