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Percentage of investment acquired by members or limited partners of limited liability company (LLC).

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The recovery percentage of the entity's surety bond claim losses that its joint venture partner is obligated to pay.

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The amount of total assets using prescribed or permitted statutory accounting practices (rather than GAAP, if different) of the state or country.

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Number of persons employed by the Entity

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Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

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Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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Percentage investment held by the managing member or general partner of the limited liability company (LLC) or limited partnership (LP).

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Percentage investment held by members or limited partners of limited liability company (LLC) or limited partnership (LP).

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Represents the number of stores.

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Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income.

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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.

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Minimum amount of purchase arrangement maturing after the fifth fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

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Minimum amount of purchase arrangement maturing in the fifth fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

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Minimum amount of purchase arrangement maturing in the fourth fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

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Minimum amount of purchase arrangement maturing in the next fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

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Minimum amount of purchase arrangement maturing in the second fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

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Minimum amount of purchase arrangement maturing in the third fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

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The amount of statutory capital and surplus (stockholders' equity) as of the balance sheet date using prescribed or permitted statutory accounting practices (rather than GAAP, if different) of the state or country.

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Contingencies and Commitments - Narrative (Detail)		3 Months Ended	12 Months Ended
Contingencies and Commitments - Narrative (Detail)	Oct. 03, 2017 USD ($) Employee Location	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2017 USD ($)
Contingencies and Commitments [Line Items]
Operating leases, rent expense			$ 1,579,000,000	$ 1,573,000,000	$ 1,516,000,000
Purchase commitments - 2018			13,000,000,000
Purchase commitments - 2019			3,800,000,000
Purchase commitments - 2020			3,100,000,000
Purchase commitments - 2021			2,600,000,000
Purchase commitments - 2022			2,400,000,000
Purchase commitments - After 2022			15,000,000,000
Estimated cost to acquire certain equity ownership interests of less than wholly-owned subsidiaries			$ 5,300,000,000
Investment Agreement and Equity Purchase Agreement [Member] \| Pilot Travel Centers LLC, , d/b/a Pilot Flying J [Member]
Contingencies and Commitments [Line Items]
Number of locations \| Location	750
Annual revenues	$ 20,000,000,000
NICO [Member] \| Purchase Agreement [Member] \| Medical Liability Mutual Insurance Company [Member]
Contingencies and Commitments [Line Items]
Statutory assets						$ 5,800,000,000
Statutory policyholders' surplus						$ 2,200,000,000
Scenario, Forecast [Member] \| NICO [Member] \| Purchase Agreement [Member] \| Medical Liability Mutual Insurance Company [Member]
Contingencies and Commitments [Line Items]
Acquisition price		$ 2,500,000,000
Berkadia Commercial Mortgage (investee) [Member]
Contingencies and Commitments [Line Items]
Voting interest in investee			50.00%
Commercial paper outstanding			$ 1,470,000,000
Berkadia Commercial Mortgage (investee) [Member] \| Commercial Paper [Member]
Contingencies and Commitments [Line Items]
Maximum outstanding balance of commercial paper borrowings			$ 1,500,000,000
Leucadia National Corporation (venture partner) [Member] \| Berkadia Commercial Mortgage (investee) [Member]
Contingencies and Commitments [Line Items]
Voting interest in investee			50.00%
Leucadia National Corporation (venture partner) [Member] \| Berkadia Commercial Mortgage (investee) [Member] \| Insurance Group [Member]
Contingencies and Commitments [Line Items]
Reimbursement rate from joint venture partner of Company's surety bond claim losses			50.00%
Haslam Family [Member] \| Investment Agreement and Equity Purchase Agreement [Member] \| Pilot Travel Centers LLC, , d/b/a Pilot Flying J [Member]
Contingencies and Commitments [Line Items]
Majority ownership percentage	50.10%
Haslam Family [Member] \| Agreement to Acquire Additional Interest in 2023 [Member] \| Pilot Travel Centers LLC, , d/b/a Pilot Flying J [Member]
Contingencies and Commitments [Line Items]
Ownership percentage	20.00%
Third Party [Member] \| Investment Agreement and Equity Purchase Agreement [Member] \| Pilot Travel Centers LLC, , d/b/a Pilot Flying J [Member]
Contingencies and Commitments [Line Items]
Ownership percentage	11.30%
Minimum [Member] \| Investment Agreement and Equity Purchase Agreement [Member] \| Pilot Travel Centers LLC, , d/b/a Pilot Flying J [Member]
Contingencies and Commitments [Line Items]
Number of team members \| Employee	27,000
Berkshire Hathaway Inc. (Parent) [Member] \| Investment Agreement and Equity Purchase Agreement [Member] \| Pilot Travel Centers LLC, , d/b/a Pilot Flying J [Member]
Contingencies and Commitments [Line Items]
Ownership percentage	38.60%
Berkshire Hathaway Inc. (Parent) [Member] \| Agreement to Acquire Additional Interest in 2023 [Member] \| Pilot Travel Centers LLC, , d/b/a Pilot Flying J [Member]
Contingencies and Commitments [Line Items]
Percentage of additional interest to be acquired	41.40%