Summary of Reconciles Pro Forma Amounts Reported Under the Prior Standard to Current Reporting - Consolidated Balance Sheets (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	[1]
STOCKHOLDERS' EQUITY:
Total stockholders' equity	$ 9,171.0		$ 8,050.5
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
Proforma Under Prior Standard | Previously Reported
STOCKHOLDERS' EQUITY:
Total stockholders' equity	9,139.7
Proforma Under Prior Standard | Truck, Parts and Other | Previously Reported
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Other current assets	264.8
Equipment on operating leases, net	1,619.3
Other noncurrent assets, net	536.9
Accounts payable, accrued expenses and other	3,119.1
Residual value guarantees and deferred revenues	1,725.0
Other liabilities	849.1
Effects of New Standard | Adjustments
STOCKHOLDERS' EQUITY:
Total stockholders' equity	31.3
Effects of New Standard | Truck, Parts and Other | Adjustments
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Other current assets	112.0
Equipment on operating leases, net	(840.7)
Other noncurrent assets, net	186.6
Accounts payable, accrued expenses and other	115.0
Residual value guarantees and deferred revenues	(892.6)
Other liabilities	$ 204.2

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