Finance Receivable Evaluated and Determined Individually and Collectively (Detail) - Financial Services - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded investment for impaired finance receivables evaluated individually	$ 44.7	$ 54.9
Allowance for impaired finance receivables determined individually	5.7	6.7
Recorded investment for finance receivables evaluated collectively	10,487.7	9,598.3
Allowance for finance receivables determined collectively	105.6	101.2
Dealer \| Wholesale
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded investment for impaired finance receivables evaluated individually	0.1	0.1
Allowance for impaired finance receivables determined individually	0.1	0.1
Recorded investment for finance receivables evaluated collectively	2,229.5	1,880.5
Allowance for finance receivables determined collectively	6.5	5.9
Dealer \| Retail
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded investment for impaired finance receivables evaluated individually	2.4	4.0
Allowance for impaired finance receivables determined individually	0.0	0.0
Recorded investment for finance receivables evaluated collectively	1,370.6	1,354.7
Allowance for finance receivables determined collectively	9.3	9.4
Customer Retail
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded investment for impaired finance receivables evaluated individually	42.2	50.8
Allowance for impaired finance receivables determined individually	5.6	6.6
Recorded investment for finance receivables evaluated collectively	6,887.6	6,363.1
Allowance for finance receivables determined collectively	$ 89.8	$ 85.9

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

+ Details

Name:	us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

+ Details

Name:	us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=pcar_FinancialServicesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=pcar_DealerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=pcar_WholesaleMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=us-gaap_RetailMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=pcar_CustomerRetailMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: