Marketable Debt Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 1,099.3	$ 1,023.5
Unrealized Gains	9.1	1.4
Unrealized Losses	0.6	4.5
Fair Value	1,107.8	1,020.4	[1]
U.S. tax-exempt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	316.8	326.0
Unrealized Gains	1.9	0.3
Unrealized Losses	0.3	1.2
Fair Value	318.4	325.1
U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	160.6	147.6
Unrealized Gains	2.0	0.2
Unrealized Losses		0.4
Fair Value	162.6	147.4
U.S. government and agency securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	126.1	98.9
Unrealized Gains	1.0	0.2
Unrealized Losses		0.4
Fair Value	127.1	98.7
Non-U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	303.0	272.5
Unrealized Gains	2.6	0.4
Unrealized Losses	0.2	1.6
Fair Value	305.4	271.3
Non-U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	65.0	55.9
Unrealized Gains	0.3	0.1
Unrealized Losses		0.1
Fair Value	65.3	55.9
Other debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	127.8	122.6
Unrealized Gains	1.3	0.2
Unrealized Losses	0.1	0.8
Fair Value	$ 129.0	$ 122.0

[1] The December 31, 2018 consolidated balance sheet has been derived from audited financial statements.