Summary of Cumulative Effect of the Changes Made for the Adoption of ASU 2014-09 (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	[1]
STOCKHOLDERS' EQUITY:
Retained earnings	$ 9,718.4	$ 8,386.2	$ 8,369.1
Truck, Parts and Other
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Other current assets	376.8	504.4	404.4
Equipment on operating leases, net	778.6	596.9	1,265.7
Other noncurrent assets, net	723.5	540.2	425.2
Accounts payable, accrued expenses and other	3,234.1	2,672.6	2,569.5
Residual value guarantees and deferred revenues	832.4	635.2	1,339.0
Other liabilities	$ 1,053.3	1,069.6	$ 939.8
ASU 2014-09 | Adjustments
STOCKHOLDERS' EQUITY:
Retained earnings		17.1
ASU 2014-09 | Truck, Parts and Other | 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

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