Significant Accounting Policies - Additional Information (Detail) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) Segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Accounting Policies [Abstract]
Reportable segments | Segment									3
Liquid investments maturity period									90 days
Goodwill	$ 117.4				$ 103.0				$ 117.4	$ 103.0
Derivative assets	57.8				118.4				57.8	118.4
Decrease in RVG assets	23,440.2				20,638.9				23,440.2	20,638.9	$ 21,109.8
Retained Earnings
Accounting Policies [Abstract]
Increase in retained earnings by cumulative effect adjustment									(19.9)
Accounting Standards Update 2016-16
Accounting Policies [Abstract]
New accounting pronouncement, cumulative effect of change on prepaid income taxes and retained earnings	(19.9)								(19.9)
Accounting Standards Update 2014-09
Accounting Policies [Abstract]
Increase in retained earnings by cumulative effect adjustment									17.0
Accounting Standards Update 2014-09 | Restatement Adjustment
Accounting Policies [Abstract]
Decrease in RVG assets	(497.0)								(497.0)
Decrease in RVG liability	(520.0)								(520.0)
New Accounting Pronouncement, Early Adoption, Effect | Retained Earnings
Accounting Policies [Abstract]
Tax effect of reclassification from AOCI to retained earnings									40.0
New Accounting Pronouncement, Early Adoption, Effect | Accumulated Other Comprehensive Income
Accounting Policies [Abstract]
Tax effect of reclassification from AOCI to retained earnings									(40.0)
Designated under hedge accounting
Accounting Policies [Abstract]
Derivative assets	57.1				113.6				57.1	113.6
Designated under hedge accounting | Interest-Rate Contracts | Other Assets
Accounting Policies [Abstract]
Derivative assets	53.3								$ 53.3
Engines manufactured by PACCAR
Accounting Policies [Abstract]
Warranty period									2 years
Trade Receivables
Accounting Policies [Abstract]
Trade and other receivables, net	962.0				734.6				$ 962.0	734.6
Other Receivables
Accounting Policies [Abstract]
Trade and other receivables, net	165.9				127.6				$ 165.9	127.6
Minimum
Accounting Policies [Abstract]
Lease and guarantee periods (in years)									2 years
Estimated useful life of equipment (in years)									4 years
Warranty period									1 year
Maximum
Accounting Policies [Abstract]
Lease and guarantee periods (in years)									5 years
Estimated useful life of equipment (in years)									9 years
Warranty period									5 years
Financial Services
Accounting Policies [Abstract]
Operating lease, rental and other revenues									$ 837.8	760.5	728.5
Depreciation and other expenses	188.8	$ 186.2	$ 172.8	$ 179.7	165.3	$ 162.6	$ 156.4	$ 150.9	$ 727.5	$ 635.2	583.7
Months contractual terms extended									5 months	4 months
Decrease in RVG assets	13,202.3				12,194.8				$ 13,202.3	$ 12,194.8	12,254.6
Decrease in RVG liability	10,119.3				9,805.1				10,119.3	9,805.1
Financial Services | Designated under hedge accounting | Interest-Rate Contracts | Other Assets
Accounting Policies [Abstract]
Derivative assets	53.3				109.7				53.3	109.7
Financial Services | Operating Lease
Accounting Policies [Abstract]
Operating lease, rental and other revenues									760.9	698.9	668.6
Depreciation and other expenses									$ 665.7	581.7	$ 536.2
Financial Services | Minimum | Loans Receivable
Accounting Policies [Abstract]
Origination cost, amortization period									36 months
Financial Services | Minimum | Loans And Leases
Accounting Policies [Abstract]
Receivable, collection period									3 years
Financial Services | Maximum | Loans Receivable
Accounting Policies [Abstract]
Origination cost, amortization period									60 months
Financial Services | Maximum | Loans And Leases
Accounting Policies [Abstract]
Receivable, collection period									5 years
Truck, Parts and Other
Accounting Policies [Abstract]
Trade and other receivables, net	1,127.9				862.2				$ 1,127.9	$ 862.2
Trade receivable days outstanding considered past due									30 days	30 days	30 days
Allowance for credit losses for Truck, Parts and Other	1.5				1.7				$ 1.5	$ 1.7
Net charge-offs									0.1	0.1	$ 0.3
Decrease in RVG assets	10,237.9				8,444.1				10,237.9	8,444.1	$ 8,855.2
Decrease in RVG liability	$ 5,270.4				$ 4,056.2				$ 5,270.4	$ 4,056.2