Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	$ (581.8)	$ (494.9)	$ (1,569.0)	$ (711.5)
Investment income	(9.0)	(8.5)	(25.8)	(20.6)
Tax (benefit) expense	179.1	148.7	483.0	478.6
Net Income (Loss)	(402.7)	(346.2)	(1,086.0)	(232.9)
Total reclassifications out of AOCI	25.4	13.8	107.1	50.9
Foreign-exchange contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	3.5	(11.4)	19.2	(15.4)
Interest-Rate Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	24.6	28.2	110.4	78.5
Derivative Contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI	21.0	10.8	94.0	39.2
Derivative Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	28.1	16.8	129.6	63.1
Tax (benefit) expense	(7.1)	(6.0)	(35.6)	(23.9)
Net Income (Loss)	21.0	10.8	94.0	39.2
Marketable Debt Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI	(0.1)	(1.8)	(0.4)	(2.6)
Marketable Debt Securities | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Investment income	(0.2)	(2.4)	(0.6)	(3.5)
Tax (benefit) expense	0.1	0.6	0.2	0.9
Net Income (Loss)	(0.1)	(1.8)	(0.4)	(2.6)
Pension Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI	4.5	4.8	13.5	14.3
Pension Plans | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	6.6	7.2	19.8	21.7
Tax (benefit) expense	(2.1)	(2.4)	(6.3)	(7.4)
Net Income (Loss)	4.5	4.8	13.5	14.3
Truck, Parts and Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net sales and revenues	(4,731.5)	(3,953.2)	(13,065.1)	(12,079.6)
Cost of sales and revenues	4,046.8	3,371.5	11,184.2	10,274.5
Selling, general and administrative	112.9	105.5	332.0	326.0
Research and development	67.0	59.2	194.1	179.6
Interest and other expense, net	3.2	1.6	3.1	4.2
Income before income taxes	(501.6)	(415.4)	(1,351.7)	(462.3)
Truck, Parts and Other | Derivative Contracts | Foreign-exchange contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net sales and revenues	(3.0)	(11.1)	13.3	(17.6)
Cost of sales and revenues	5.5	0.0	4.3	0.6
Interest and other expense, net	1.0	(0.3)	1.6	1.6
Truck, Parts and Other | Derivative Contracts | Interest-Rate Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net sales and revenues	0.0	0.0	0.0	0.0
Cost of sales and revenues	0.0	0.0	0.0	0.0
Interest and other expense, net	0.0	0.0	0.0	0.0
Truck, Parts and Other | Actuarial loss | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of sales and revenues	3.1	3.4	9.2	10.3
Selling, general and administrative	3.0	3.2	9.1	9.7
Income before income taxes	6.1	6.6	18.3	20.0
Truck, Parts and Other | Prior service costs | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of sales and revenues	0.2	0.2	0.7	0.7
Selling, general and administrative	0.1	0.1	0.2	0.2
Income before income taxes	0.3	0.3	0.9	0.9
Financial Services
Accumulated Other Comprehensive Income (Loss) [Line Items]
Selling, general and administrative	27.8	25.3	79.3	74.9
Interest and other borrowing expenses	38.3	32.2	109.8	95.1
Income before income taxes	(71.2)	(71.0)	(191.5)	(228.6)
Financial Services | Derivative Contracts | Foreign-exchange contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other borrowing expenses	0.0	0.0	0.0	0.0
Financial Services | Derivative Contracts | Interest-Rate Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other borrowing expenses	24.6	28.2	110.4	78.5
Financial Services | Actuarial loss | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Selling, general and administrative	$ 0.2	$ 0.3	$ 0.6	$ 0.8