Summary of Reconciles Pro Forma Amounts Reported Under the Prior Standard to Current Reporting - Consolidated Balance Sheets (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	[1]
STOCKHOLDERS' EQUITY:
Total stockholders' equity	$ 8,779.4		$ 8,050.5
Truck, Parts and Other
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Other current assets	492.9	$ 504.4	404.4
Equipment on operating leases, net	711.0	596.9	1,265.7
Other noncurrent assets, net	673.4	540.2	425.2
Accounts payable, accrued expenses and other	3,096.0	2,672.6	2,569.5
Residual value guarantees and deferred revenues	758.2	635.2	1,339.0
Other liabilities	1,115.3	$ 1,069.6	$ 939.8
Proforma Under Prior Standard | Previously Reported
STOCKHOLDERS' EQUITY:
Total stockholders' equity	8,757.6
Proforma Under Prior Standard | Truck, Parts and Other | Previously Reported
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Other current assets	383.9
Equipment on operating leases, net	1,426.3
Other noncurrent assets, net	543.5
Accounts payable, accrued expenses and other	2,983.9
Residual value guarantees and deferred revenues	1,513.8
Other liabilities	970.0
Effects of New Standard | Adjustments
STOCKHOLDERS' EQUITY:
Total stockholders' equity	21.8
Effects of New Standard | Truck, Parts and Other | Adjustments
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Other current assets	109.0
Equipment on operating leases, net	(715.3)
Other noncurrent assets, net	129.9
Accounts payable, accrued expenses and other	112.1
Residual value guarantees and deferred revenues	(755.6)
Other liabilities	$ 145.3

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