Summary of Reconciles Pro Forma Amounts Reported Under the Prior Standard to Current Reporting - Consolidated Balance Sheets (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 | Proforma Under Prior Standard | Previously Reported
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Other current assets	294.0
Equipment on operating leases, net	1,359.7
Other noncurrent assets, net	544.4
Accounts payable, accrued expenses and other	2,911.0
Residual value guarantees and deferred revenues	1,441.5
Other liabilities	980.3
STOCKHOLDERS' EQUITY:
Retained earnings	8,789.1
Truck, Parts and Other | Effects of New Standard | Adjustments
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Other current assets	102.6
Equipment on operating leases, net	(714.5)
Other noncurrent assets, net	141.4
Accounts payable, accrued expenses and other	105.8
Residual value guarantees and deferred revenues	(754.7)
Other liabilities	157.4
STOCKHOLDERS' EQUITY:
Retained earnings	$ 21.0

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