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
	Mar. 31, 2018	Mar. 31, 2017
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Income Before Income Taxes	$ 669.4	$ 448.2
Income taxes	157.3	137.9
Net income	512.1	310.3
Comprehensive Income	587.4	380.4
Proforma Under Prior Standard | Previously Reported
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Income Before Income Taxes	665.0
Income taxes	156.2
Net income	508.8
Comprehensive Income	583.5
Effects of New Standard | Adjustments
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Income Before Income Taxes	4.4
Income taxes	1.1
Net income	3.3
Comprehensive Income	3.9
Truck, Parts and Other
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net sales and revenues	5,321.8	3,935.7
Cost of sales and revenues	4,535.5	3,390.9
Income Before Income Taxes	591.9	$ 383.3
Truck, Parts and Other | Proforma Under Prior Standard | Previously Reported
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net sales and revenues	5,314.2
Cost of sales and revenues	4,532.3
Income Before Income Taxes	587.5
Truck, Parts and Other | Effects of New Standard | Adjustments
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net sales and revenues	7.6
Cost of sales and revenues	3.2
Income Before Income Taxes	$ 4.4