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Fair Value Measurements (Narrative) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
(Decrease) increase in fair value of marketable securities | $ (1,700) | $ 3,600
Non-financial assets and liabilities measured at fair value on a nonrecurring basis | $ 0 | $ 0

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- Definition Represents the aggregate of the non-financial assets and/or liabilities reported on the balance sheet at period end measured at fair value on a nonrecurring basis. Non-financial assets and liabilities may include assets acquired in a business combination or property and equipment that are determined to be impaired. + References No definition available. + Details
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Namespace Prefix: | orly_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
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- Definition Amount of increase (decrease) of investment in debt and equity securities, measured at fair value with changes in fair value recognized in net income. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 19 -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3444-108585 + Details
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