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								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
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Income Before Income Taxes									$ 2,810.2	$ 2,173.3	$ 1,130.4
Income taxes									615.1	498.1	608.7
Net income	$ 578.1	$ 545.3	$ 559.6	$ 512.1	$ 589.2	$ 402.7	$ 373.0	$ 310.3	2,195.1	1,675.2	521.7
Comprehensive Income									1,923.4	2,009.7	410.6
Proforma Under Prior Standard | Previously Reported
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Income Before Income Taxes									2,778.2
Income taxes									607.1
Net income									2,171.1
Comprehensive Income									1,900.7
Effects of New Standard | Adjustments
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Income Before Income Taxes									32.0
Income taxes									8.0
Net income									24.0
Comprehensive Income									22.7
Truck, Parts and Other
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net sales and revenues	5,932.7	5,416.9	5,467.2	5,321.8	5,122.4	4,731.5	4,397.9	3,935.7	22,138.6	18,187.5	15,846.6
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 | Proforma Under Prior Standard | Previously Reported
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net sales and revenues									21,900.5
Cost of sales and revenues									18,718.9
Income Before Income Taxes									2,411.4
Truck, Parts and Other | Effects of New Standard | Adjustments
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net sales and revenues									238.1
Cost of sales and revenues									206.1
Income Before Income Taxes									$ 32.0