Significant Accounting Policies - Additional Information (Detail)	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) Segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Accounting Policies [Abstract]
Reportable segments | Segment									3
Term used to calculate historical weighted average return rate									5 years
Estimated value of trucks to be returned	$ 473,000,000.0				$ 319,800,000				$ 473,000,000.0	$ 319,800,000
Estimated sales returned liabilities									503,400,000	329,300,000
Payment to acquire commitment value									894,500,000
Estimated value of returned goods	56,300,000				49,000,000.0				56,300,000	49,000,000.0
Sales returns liabilities in estimated returns									$ 126,300,000	104,500,000
Liquid investments maturity period									90 days
Goodwill	109,100,000				112,000,000.0				$ 109,100,000	112,000,000.0
Derivative assets	56,300,000				94,700,000				56,300,000	94,700,000
Sales type finance leases reclassified to other assets, net, increase										30,100,000	$ 58,400,000
Direct loans reclassified to trade and other receivables, increase (decrease)										(57,100,000)	13,500,000
Unearned interest on finance leases					387,500,000					387,500,000
ASU 2016-02
Accounting Policies [Abstract]
Cash originations from sales-type leases	224,300,000								224,300,000
Cash receipts from sales-type leases	197,000,000.0								197,000,000.0
ASU 2016-13
Accounting Policies [Abstract]
Increase in finance receivable allowance for credit losses									6,200,000
Cumulative effect of change in accounting principles									4,600,000
Designated under hedge accounting
Accounting Policies [Abstract]
Derivative assets	55,900,000				93,400,000				55,900,000	93,400,000
Designated under hedge accounting | Interest-Rate Contracts | Other Assets
Accounting Policies [Abstract]
Derivative assets	56,300,000								56,300,000
Trade Receivables
Accounting Policies [Abstract]
Trade and other receivables, net	1,055,000,000.0				1,103,600,000				1,055,000,000.0	1,103,600,000
Other Receivables
Accounting Policies [Abstract]
Trade and other receivables, net	251,100,000				210,800,000				251,100,000	210,800,000
Financial Services
Accounting Policies [Abstract]
Total operating lease income									798,200,000
Depreciation and other expenses	241,900,000	$ 195,300,000	$ 183,600,000	$ 177,400,000	177,600,000	$ 178,500,000	$ 185,500,000	$ 186,400,000
Financing receivable, recorded investment, 90 days past due and still accruing	0				0				$ 0	$ 0
Months contractual terms extended									5 months	6 months
Financial Services | Designated under hedge accounting | Interest-Rate Contracts | Other Assets
Accounting Policies [Abstract]
Derivative assets	45,800,000				84,500,000				$ 45,800,000	$ 84,500,000
Financial Services | Operating Lease
Accounting Policies [Abstract]
Total operating lease income									798,200,000	797,100,000	760,900,000
Depreciation and other expenses									721,600,000	686,900,000	$ 665,700,000
Truck, Parts and Other
Accounting Policies [Abstract]
Trade and other receivables, net	1,306,100,000				1,314,400,000				$ 1,306,100,000	$ 1,314,400,000
Trade receivable days outstanding considered past due									30 days	30 days	30 days
Allowance for credit losses for Truck, Parts and Other	$ 1,000,000.0				$ 1,000,000.0				$ 1,000,000.0	$ 1,000,000.0
Net charge-offs									$ 300,000	100,000	$ 100,000
Engines manufactured by PACCAR
Accounting Policies [Abstract]
Warranty period									2 years
Trucks | Truck Sales
Accounting Policies [Abstract]
Total operating lease income									$ 159,700,000	$ 152,600,000
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