Quarterly Results (Unaudited) (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information [Line Items]
Revenues									$ 18,728.5	$ 25,599.7	$ 23,495.7
Net income	$ 405.8	$ 385.5	$ 147.7	$ 359.4	$ 531.3	$ 607.9	$ 619.7	$ 629.0	$ 1,298.4	$ 2,387.9	$ 2,195.1
Net Income Per Share
Basic	$ 1.17	$ 1.11	$ 0.43	$ 1.04	$ 1.53	$ 1.75	$ 1.79	$ 1.81	$ 3.74	$ 6.88	$ 6.25
Diluted	$ 1.17	$ 1.11	$ 0.43	$ 1.03	$ 1.53	$ 1.75	$ 1.78	$ 1.81	$ 3.74	$ 6.87	$ 6.24
Truck, Parts and Other
Quarterly Financial Information [Line Items]
Net sales and revenues	$ 5,136.0	$ 4,538.4	$ 2,701.9	$ 4,778.0	$ 5,710.9	$ 6,004.2	$ 6,266.5	$ 6,138.1	$ 17,154.3	$ 24,119.7
Cost of sales and revenues	4,487.0	3,958.3	2,441.5	4,189.6	4,890.0	5,106.8	5,341.7	5,217.1
Research and development	71.7	64.7	66.5	71.0	83.6	82.2	82.5	78.3
Financial Services
Quarterly Financial Information [Line Items]
Revenues	432.6	397.6	360.3	383.7	406.3	362.8	361.4	349.5
Interest and other borrowing expenses	43.6	45.2	46.9	56.4	57.5	59.6	60.0	53.4
Depreciation and other expenses	$ 292.0	$ 262.5	$ 224.1	$ 229.4	$ 241.9	$ 195.3	$ 183.6	$ 177.4