Summary of Effects on the Consolidated Statements of Comprehensive Income by Retrospective Application of ASU 2017-07 (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Income Before Income Taxes									$ 2,810.2	$ 2,173.3	$ 1,130.4
Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues	$ 5,088.6	$ 4,653.6	$ 4,647.3	$ 4,535.5	$ 4,418.4	$ 4,055.6	$ 3,764.0	$ 3,390.9	18,925.0
Income Before Income Taxes									2,443.4	1,876.3	797.1
Truck, Parts and Other | ASU 2017-07
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues										15,628.9	13,533.6
Selling, general and administrative										464.0	442.6
Interest and other (income), net										(46.4)	(6.9)
Income Before Income Taxes										1,876.3	797.1
Truck, Parts and Other | ASU 2017-07 | Previously Reported
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues										15,593.7	13,517.7
Selling, general and administrative										449.5	440.8
Interest and other (income), net										5.6	11.6
Income Before Income Taxes										1,874.0	796.3
Truck, Parts and Other | ASU 2017-07 | Effect of Change
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues										35.2	15.9
Selling, general and administrative										14.5	1.8
Interest and other (income), net										(52.0)	(18.5)
Income Before Income Taxes										2.3	0.8
Financial Services
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Income Before Income Taxes									$ 305.9	261.7	305.7
Financial Services | ASU 2017-07
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Selling, general and administrative										107.8	100.2
Income Before Income Taxes										261.7	305.7
Financial Services | ASU 2017-07 | Previously Reported
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Selling, general and administrative										105.5	99.4
Income Before Income Taxes										264.0	306.5
Financial Services | ASU 2017-07 | Effect of Change
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Selling, general and administrative										2.3	0.8
Income Before Income Taxes										$ (2.3)	$ (0.8)