Summary of Cumulative Effect of the Changes Made for the Adoption of ASU 2014-09 (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
STOCKHOLDERS' EQUITY:
Retained earnings	$ 8,810.1		$ 8,369.1	[1]
Truck, Parts and Other
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Other current assets	396.6	$ 504.4	404.4	[1]
Equipment on operating leases, net	645.2	596.9	1,265.7	[1]
Other noncurrent assets, net	685.8	540.2	425.2	[1]
Accounts payable, accrued expenses and other	3,016.8	2,672.6	2,569.5	[1]
Residual value guarantees and deferred revenues	686.8	635.2	1,339.0	[1]
Other liabilities	1,137.7	1,069.6	939.8	[1]
STOCKHOLDERS' EQUITY:
Retained earnings	$ 8,810.1	8,386.2	$ 8,369.1
Truck, Parts and Other | ASU 2014-09 | Adjustments
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Other current assets		100.0
Equipment on operating leases, net		(668.8)
Other noncurrent assets, net		115.0
Accounts payable, accrued expenses and other		103.1
Residual value guarantees and deferred revenues		(703.8)
Other liabilities		129.8
STOCKHOLDERS' EQUITY:
Retained earnings		$ 17.1

[1] The December 31, 2017 consolidated balance sheet has been derived from audited financial statements.