Marketable Debt Securities (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 1,108.9	$ 1,023.5
Unrealized Gains	8.8	1.4
Unrealized Losses	0.4	4.5
Fair Value	1,117.3	1,020.4	[1]
U.S. tax-exempt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	329.3	326.0
Unrealized Gains	2.0	0.3
Unrealized Losses	0.1	1.2
Fair Value	331.2	325.1
U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	162.5	147.6
Unrealized Gains	1.8	0.2
Unrealized Losses		0.4
Fair Value	164.3	147.4
U.S. government and agency securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	106.2	98.9
Unrealized Gains	1.0	0.2
Unrealized Losses		0.4
Fair Value	107.2	98.7
Non-U.S. corporate securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	310.7	272.5
Unrealized Gains	2.5	0.4
Unrealized Losses	0.2	1.6
Fair Value	313.0	271.3
Non-U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	69.9	55.9
Unrealized Gains	0.3	0.1
Unrealized Losses		0.1
Fair Value	70.2	55.9
Other debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	130.3	122.6
Unrealized Gains	1.2	0.2
Unrealized Losses	0.1	0.8
Fair Value	$ 131.4	$ 122.0

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