Quarterly Results (Unaudited) (Detail) $ / shares in Units, € in Millions, $ in Millions		3 Months Ended												6 Months Ended		12 Months Ended
		Dec. 31, 2017 USD ($) $ / shares		Sep. 30, 2017 USD ($) $ / shares		Jun. 30, 2017 USD ($) $ / shares		Mar. 31, 2017 USD ($) $ / shares		Dec. 31, 2016 USD ($) $ / shares	Sep. 30, 2016 USD ($) $ / shares	Jun. 30, 2016 USD ($) $ / shares	Mar. 31, 2016 USD ($) $ / shares	Jun. 30, 2016 USD ($)	Jun. 30, 2016 EUR (€)	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares	Dec. 31, 2015 USD ($) $ / shares
Quarterly Financial Information [Line Items]
Net income		$ 589.2		$ 402.7		$ 373.0		$ 310.3		$ 288.8	$ 346.2	$ 481.3	$ (594.6)			$ 1,675.2	$ 521.7	$ 1,604.0
Adjusted Net Income	[1]	$ 415.8		$ 0.0		$ 0.0		$ 0.0				$ 371.7	$ 348.0
Net Income Per Share
Basic | $ / shares		$ 1.67	[2]	$ 1.14	[2]	$ 1.06	[2]	$ 0.88	[2]	$ 0.82	$ 0.99	$ 1.37	$ (1.69)			$ 4.76	$ 1.49	$ 4.52
Diluted | $ / shares		1.67	[2]	1.14	[2]	1.06	[2]	0.88	[2]	$ 0.82	$ 0.98	1.37	(1.69)			$ 4.75	$ 1.48	$ 4.51
Adjusted Diluted | $ / shares	[1]	$ 1.18	[2]	$ 0.00	[2]	$ 0.00	[2]	$ 0.00	[2]			$ 1.06	$ 0.99
Truck, Parts and Other
Quarterly Financial Information [Line Items]
Net sales and revenues		$ 5,122.4		$ 4,731.5		$ 4,397.9		$ 3,935.7		$ 3,767.0	$ 3,953.2	$ 4,115.8	$ 4,010.6			$ 18,187.5	$ 15,846.6	$ 17,942.8
Cost of sales and revenues		4,409.5		4,046.8		3,755.2		3,382.2		3,243.2	3,371.5	3,489.4	3,413.6			15,593.7	13,517.7	15,292.1
Research and development		70.6		67.0		66.1		61.0		67.6	59.2	60.8	59.6			264.7	247.2	239.8
European Commission charge												(109.6)	942.6	$ 833.0	€ 752.7	0.0	833.0	0.0
Financial Services
Quarterly Financial Information [Line Items]
Revenues		332.2		328.2		306.3		302.2		303.7	296.2	297.4	289.4			1,268.9	1,186.7	1,172.3
Interest and other borrowing expenses		39.8		38.3		37.4		34.1		32.1	32.2	32.6	30.3			149.6	127.2	118.0
Depreciation and other expenses		$ 188.8		$ 186.2		$ 172.8		$ 179.7		$ 165.3	$ 162.6	$ 156.4	$ 150.9			$ 727.5	$ 635.2	$ 583.7

[1] See Reconciliation of GAAP to Non-GAAP Financial Measures for 2017 and 2016 on page 33.

[2] The sum of quarterly per share amounts may not equal per share amounts reported for year-to-date periods. This is due to changes in the number of weighted shares outstanding and the effects of rounding for each period.