Summary of Effects on the Consolidated Statement of Comprehensive Income by Retrospective Application of ASU 2017-07 (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Income Before Income Taxes	$ 669.4	$ 448.2
Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues	4,535.5	3,390.9
Selling, general and administrative	137.1	114.5
Interest and other (income), net	18.7	14.0
Income Before Income Taxes	591.9	383.3
Financial Services
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Selling, general and administrative	31.1	25.7
Income Before Income Taxes	$ 67.5	56.8
ASU 2017-07 \| Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues		3,390.9
Selling, general and administrative		114.5
Interest and other (income), net		(14.0)
Income Before Income Taxes		383.3
ASU 2017-07 \| Truck, Parts and Other \| Previously Reported
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues		3,382.2
Selling, general and administrative		111.3
Interest and other (income), net		(1.6)
Income Before Income Taxes		382.8
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		8.7
Selling, general and administrative		3.2
Interest and other (income), net		(12.4)
Income Before Income Taxes		0.5
ASU 2017-07 \| Financial Services
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Selling, general and administrative		25.7
Income Before Income Taxes		56.8
ASU 2017-07 \| Financial Services \| Previously Reported
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Selling, general and administrative		25.2
Income Before Income Taxes		57.3
ASU 2017-07 \| Financial Services \| Effect of Change
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Selling, general and administrative		0.5
Income Before Income Taxes		$ (0.5)

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+ References

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