Quarterly Results (Unaudited) (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information [Line Items]
Revenues									$ 25,599.7	$ 23,495.7	$ 19,456.4
Net income	$ 531.3	$ 607.9	$ 619.7	$ 629.0	$ 578.1	$ 545.3	$ 559.6	$ 512.1	$ 2,387.9	$ 2,195.1	$ 1,675.2
Net Income Per Share
Basic	$ 1.53	$ 1.75	$ 1.79	$ 1.81	$ 1.66	$ 1.55	$ 1.59	$ 1.45	$ 6.88	$ 6.25	$ 4.76
Diluted	$ 1.53	$ 1.75	$ 1.78	$ 1.81	$ 1.65	$ 1.55	$ 1.59	$ 1.45	$ 6.87	$ 6.24	$ 4.75
Truck, Parts and Other
Quarterly Financial Information [Line Items]
Net sales and revenues	$ 5,710.9	$ 6,004.2	$ 6,266.5	$ 6,138.1	$ 5,932.7	$ 5,416.9	$ 5,467.2	$ 5,321.8	$ 24,119.7	$ 22,138.6
Cost of sales and revenues	4,890.0	5,106.8	5,341.7	5,217.1	5,088.6	4,653.6	4,647.3	4,535.5
Research and development	83.6	82.2	82.5	78.3	80.5	72.9	76.7	76.0
Financial Services
Quarterly Financial Information [Line Items]
Revenues	406.3	362.8	361.4	349.5	347.0	339.9	338.0	332.2
Interest and other borrowing expenses	57.5	59.6	60.0	53.4	50.9	49.0	45.7	41.3
Depreciation and other expenses	$ 241.9	$ 195.3	$ 183.6	$ 177.4	$ 177.6	$ 178.5	$ 185.5	$ 186.4