Significant Accounting Policies - Additional Information (Detail) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) Segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2019 USD ($)
Accounting Policies [Abstract]
Reportable segments | Segment									3
Term used to calculate historical weighted average return rate									4 years
Estimated value of trucks to be returned	$ 319.8								$ 319.8
Estimated sales returned liabilites									329.3
Guaranteed value for commitment to acquire trucks	705.9								705.9
Estimated value of parts to be retuened	49.0								49.0
Sales returns liabilities in estimated returns.									104.5
Decrease in parts sales due to change in reserve balance									$ 21.0
Liquid investments maturity period									90 days
Goodwill	112.0				$ 117.4				$ 112.0	$ 117.4
Derivative assets	94.7				57.8				$ 94.7	57.8
Revenue recognition for estimated returns, description									Return rates are estimated by using a historical weighted average return rate over a four-year period.
Accounting Standards Update 2018-02
Accounting Policies [Abstract]
Reclassification from AOCI to retained earnings due to early adoption									$ 33.2
Accounting Standards Update 2016-02
Accounting Policies [Abstract]
Cash receipts from direct financing leases									1.0
Cash originations from sales-type leases	159.4								159.4
Cash receipts from sales-type leases	189.5								189.5
Accounting Standards Update 2016-02 | Subsequent Event
Accounting Policies [Abstract]
Right-of-use asset												$ 45.0
Lease liabilities												$ 45.0
Designated under hedge accounting
Accounting Policies [Abstract]
Derivative assets	93.4				57.1				93.4	57.1
Designated under hedge accounting | Interest-Rate Contracts | Other Assets
Accounting Policies [Abstract]
Derivative assets	84.5								$ 84.5
Engines manufactured by PACCAR
Accounting Policies [Abstract]
Warranty period									2 years
Trade Receivables
Accounting Policies [Abstract]
Trade and other receivables, net	1,103.6				962.0				$ 1,103.6	962.0
Other Receivables
Accounting Policies [Abstract]
Trade and other receivables, net	210.8				165.9				$ 210.8	$ 165.9
Financial Services
Accounting Policies [Abstract]
Depreciation and other expenses	177.6	$ 178.5	$ 185.5	$ 186.4	188.8	$ 186.2	$ 172.8	$ 179.7
Months contractual terms extended									6 months	5 months
Financial Services | Designated under hedge accounting | Interest-Rate Contracts | Other Assets
Accounting Policies [Abstract]
Derivative assets	84.5				53.3				$ 84.5	$ 53.3
Financial Services | Operating Lease
Accounting Policies [Abstract]
Operating lease, rental and other revenues									797.1	760.9	$ 698.9
Depreciation and other expenses									686.9	665.7	$ 581.7
Truck, Parts and Other
Accounting Policies [Abstract]
Trade and other receivables, net	1,314.4				1,127.9				$ 1,314.4	$ 1,127.9
Trade receivable days outstanding considered past due									30 days	30 days	30 days
Allowance for credit losses for Truck, Parts and Other	$ 1.0				$ 1.5				$ 1.0	$ 1.5
Net charge-offs									$ 0.1	$ 0.1	$ 0.1
Maximum
Accounting Policies [Abstract]
Warranty period									5 years
Lease and guarantee periods (in years)									5 years
Estimated useful life of equipment (in years)									9 years
Maximum | Financial Services | Loans Receivable
Accounting Policies [Abstract]
Origination cost, amortization period									60 months
Maximum | Financial Services | Loans and Leases
Accounting Policies [Abstract]
Receivable, collection period									5 years
Minimum
Accounting Policies [Abstract]
Warranty period									1 year
Lease and guarantee periods (in years)									3 years
Estimated useful life of equipment (in years)									3 years
Minimum | Financial Services | Loans Receivable
Accounting Policies [Abstract]
Origination cost, amortization period									36 months
Minimum | Financial Services | Loans and Leases
Accounting Policies [Abstract]
Receivable, collection period									3 years