Summary of Reconciles Pro Forma Amounts Reported Under the Prior Standard to Current Reporting - Consolidated Statements of Comprehensive Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Income Before Income Taxes	$ 668.8	$ 581.8	$ 2,057.8	$ 1,569.0
Income taxes	123.5	179.1	440.8	483.0
Net income	545.3	402.7	1,617.0	1,086.0
Comprehensive Income	544.0	$ 511.7	1,515.6	$ 1,397.9
Proforma Under Prior Standard | Previously Reported
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Income Before Income Taxes	655.9		2,037.7
Income taxes	120.3		435.8
Net income	535.6		1,601.9
Comprehensive Income	534.5		1,501.4
Effects of New Standard | Adjustments
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Income Before Income Taxes	12.9		20.1
Income taxes	3.2		5.0
Net income	9.7		15.1
Comprehensive Income	9.5		14.2
Truck, Parts and Other
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net sales and revenues	5,416.9		16,205.9
Cost of sales and revenues	4,653.6		13,836.4
Income Before Income Taxes	573.6		1,798.1
Truck, Parts and Other | Proforma Under Prior Standard | Previously Reported
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net sales and revenues	5,298.1		16,065.4
Cost of sales and revenues	4,547.7		13,716.0
Income Before Income Taxes	560.7		1,778.0
Truck, Parts and Other | Effects of New Standard | Adjustments
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net sales and revenues	118.8		140.5
Cost of sales and revenues	105.9		120.4
Income Before Income Taxes	$ 12.9		$ 20.1