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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Income Before Income Taxes	$ 719.6	$ 539.0	$ 1,389.0	$ 987.2
Income taxes	160.0	166.0	317.3	303.9
Net income	559.6	373.0	1,071.7	683.3
Comprehensive Income	384.2	$ 505.8	971.6	$ 886.2
Proforma Under Prior Standard | Previously Reported
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Income Before Income Taxes	716.8		1,381.8
Income taxes	159.3		315.5
Net income	557.5		1,066.3
Comprehensive Income	383.4		966.9
Effects of New Standard | Adjustments
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Income Before Income Taxes	2.8		7.2
Income taxes	0.7		1.8
Net income	2.1		5.4
Comprehensive Income	0.8		4.7
Truck, Parts and Other
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net sales and revenues	5,467.2		10,789.0
Cost of sales and revenues	4,647.3		9,182.8
Income Before Income Taxes	632.6		1,224.5
Truck, Parts and Other | Proforma Under Prior Standard | Previously Reported
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net sales and revenues	5,453.1		10,767.3
Cost of sales and revenues	4,636.0		9,168.3
Income Before Income Taxes	629.8		1,217.3
Truck, Parts and Other | Effects of New Standard | Adjustments
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net sales and revenues	14.1		21.7
Cost of sales and revenues	11.3		14.5
Income Before Income Taxes	$ 2.8		$ 7.2