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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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Income Before Income Taxes	$ 719.6	$ 539.0	$ 1,389.0	$ 987.2
Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues	4,647.3		9,182.8
Income Before Income Taxes	$ 632.6		$ 1,224.5
ASU 2017-07 \| Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues		3,764.0		7,154.9
Selling, general and administrative		111.8		226.3
Interest and other (income), net		(11.9)		(25.9)
Income Before Income Taxes		467.9		851.2
ASU 2017-07 \| Truck, Parts and Other \| Previously Reported
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues		3,755.2		7,137.4
Selling, general and administrative		107.8		219.1
Interest and other (income), net		1.5		(0.1)
Income Before Income Taxes		467.3		850.1
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		17.5
Selling, general and administrative		4.0		7.2
Interest and other (income), net		(13.4)		(25.8)
Income Before Income Taxes		0.6		1.1
ASU 2017-07 \| Financial Services
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Selling, general and administrative		26.9		52.6
Income Before Income Taxes		62.4		119.2
ASU 2017-07 \| Financial Services \| Previously Reported
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Selling, general and administrative		26.3		51.5
Income Before Income Taxes		63.0		120.3
ASU 2017-07 \| Financial Services \| Effect of Change
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Selling, general and administrative		0.6		1.1
Income Before Income Taxes		$ (0.6)		$ (1.1)