Marketable Debt Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,460.1	$ 1,445.3
Unrealized Gains	6.5	3.9
Unrealized Losses	0.5	1.1
Fair Value	1,466.1	1,448.1	[1]
U.S. tax-exempt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	490.0	505.0
Unrealized Gains	1.7	0.7
Unrealized Losses	0.1	0.3
Fair Value	491.6	505.4
U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	79.8	76.7
Unrealized Gains	0.3	0.1
Unrealized Losses	0.1	0.1
Fair Value	80.0	76.7
U.S. government and agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16.3	15.7
Unrealized Gains	0.3	0.1
Unrealized Losses		0.1
Fair Value	16.6	15.7
Non-U.S. corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	600.1	585.6
Unrealized Gains	2.4	1.8
Unrealized Losses	0.2	0.4
Fair Value	602.3	587.0
Non-U.S. government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	171.3	192.7
Unrealized Gains	1.4	1.1
Unrealized Losses		0.1
Fair Value	172.7	193.7
Other debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	102.6	69.6
Unrealized Gains	0.4	0.1
Unrealized Losses	0.1	0.1
Fair Value	$ 102.9	$ 69.6

[1] The December 31, 2015 consolidated balance sheet has been derived from audited financial statements.