Significant Accounting Policies - Additional Information (Detail)	3 Months Ended								12 Months Ended
	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) Segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Accounting Policies [Abstract]
Reportable segments | Segment									3
Term used to calculate historical weighted average return rate									7 years
Estimated value of trucks to be returned	$ 627,900,000				$ 473,000,000.0				$ 627,900,000	$ 473,000,000.0
Estimated sales returned liabilities									664,100,000	503,400,000
Payment to acquire commitment value									1,184,700,000
Deferred revenue	795,800,000				801,400,000				795,800,000	801,400,000
Estimated value of returned goods	62,200,000				56,300,000				$ 62,200,000	56,300,000
Liquid investments maturity period									90 days
Goodwill	118,800,000				109,100,000				$ 118,800,000	109,100,000
Derivative assets	$ 20,500,000				56,300,000				$ 20,500,000	56,300,000
ASU 2016-13
Accounting Policies [Abstract]
Change in accounting principle, accounting standards update, adopted [true false]	true								true
Change in accounting principle, accounting standards update adoption date	Jan. 01, 2020								Jan. 01, 2020
ASU 2020-04
Accounting Policies [Abstract]
Change in accounting principle, accounting standards update, adopted [true false]	true								true
Change in accounting principle, accounting standards update adoption date	Oct. 01, 2020								Oct. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect [true false]	true								true
ASU 2018-13
Accounting Policies [Abstract]
Change in accounting principle, accounting standards update, adopted [true false]	true								true
Change in accounting principle, accounting standards update adoption date	Jan. 01, 2020								Jan. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect [true false]	true								true
ASU 2018-15
Accounting Policies [Abstract]
Change in accounting principle, accounting standards update, adopted [true false]	true								true
Change in accounting principle, accounting standards update adoption date	Jan. 01, 2020								Jan. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect [true false]	true								true
ASU 2019-12
Accounting Policies [Abstract]
Change in accounting principle, accounting standards update, adopted [true false]	true								true
Change in accounting principle, accounting standards update adoption date	Jan. 01, 2020								Jan. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect [true false]	true								true
ASU 2018-14
Accounting Policies [Abstract]
Change in accounting principle, accounting standards update adoption date	Jan. 01, 2021								Jan. 01, 2021
Change in accounting principle, accounting standards update, immaterial effect [true false]	true								true
Designated under hedge accounting
Accounting Policies [Abstract]
Derivative assets	$ 19,100,000				55,900,000				$ 19,100,000	55,900,000
Designated under hedge accounting | Interest-Rate Contracts | Other Assets
Accounting Policies [Abstract]
Derivative assets	20,500,000								20,500,000
Trade Receivables
Accounting Policies [Abstract]
Trade and other receivables, net	942,700,000				1,055,000,000.0				942,700,000	1,055,000,000.0
Other Receivables
Accounting Policies [Abstract]
Trade and other receivables, net	254,800,000				251,100,000				254,800,000	251,100,000
Financial Services
Accounting Policies [Abstract]
Total operating lease income									802,300,000	798,200,000
Depreciation and other expenses	292,000,000.0	$ 262,500,000	$ 224,100,000	$ 229,400,000	241,900,000	$ 195,300,000	$ 183,600,000	$ 177,400,000
Financing receivable, recorded investment, 90 days past due and still accruing	0				0				$ 0	$ 0
Months contractual terms extended									3 months	5 months
Financial Services | Designated under hedge accounting | Interest-Rate Contracts | Other Assets
Accounting Policies [Abstract]
Derivative assets	16,400,000				45,800,000				$ 16,400,000	$ 45,800,000
Financial Services | Operating Lease
Accounting Policies [Abstract]
Total operating lease income									802,300,000	798,200,000	$ 797,100,000
Depreciation and other expenses									776,500,000	721,600,000	$ 686,900,000
Truck, Parts and Other
Accounting Policies [Abstract]
Sales returns liabilities in estimated returns	337,300,000				226,000,000.0				337,300,000	226,000,000.0
Trade and other receivables, net	1,197,500,000				1,306,100,000				$ 1,197,500,000	$ 1,306,100,000
Trade receivable days outstanding considered past due									30 days	30 days	30 days
Allowance for credit losses for Truck, Parts and Other	600,000				600,000				$ 600,000	$ 600,000
Net charge-offs									$ 0	300,000	$ 100,000
Engines manufactured by PACCAR
Accounting Policies [Abstract]
Warranty period									2 years
Trucks | Truck Sales
Accounting Policies [Abstract]
Total operating lease income									$ 104,200,000	159,700,000	$ 152,600,000
Parts Subsegment
Accounting Policies [Abstract]
Sales returns liabilities in estimated returns	$ 142,000,000.0				$ 126,300,000				$ 142,000,000.0	$ 126,300,000
Maximum
Accounting Policies [Abstract]
Warranty period									5 years
Lease and guarantee periods (in years)									5 years
Estimated useful lives of equipment									10 years
Maximum | Financial Services
Accounting Policies [Abstract]
Operating lease term	5 years								5 years
Maximum | Financial Services | Loans and Leases
Accounting Policies [Abstract]
Contractual terms of retails loans and finance leases									5 years
Maximum | Trucks | Financial Services
Accounting Policies [Abstract]
Finance leases, lease term									5 years
Minimum
Accounting Policies [Abstract]
Warranty period									1 year
Lease and guarantee periods (in years)									3 years
Estimated useful lives of equipment									3 years
Minimum | Financial Services
Accounting Policies [Abstract]
Operating lease term	3 years								3 years
Minimum | Financial Services | Loans and Leases
Accounting Policies [Abstract]
Contractual terms of retails loans and finance leases									3 years
Minimum | Trucks | Financial Services
Accounting Policies [Abstract]
Finance leases, lease term									3 years