Retroactive reinsurance contracts - Narrative (Detail) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Effects of Reinsurance [Line Items]
Unpaid losses and loss adjustment expenses under retroactive reinsurance contracts	$ 41,364	$ 41,935	$ 41,834	$ 42,937
Deferred charges under retroactive reinsurance contracts	13,359	14,451	14,104	$ 15,278
Retroactive Reinsurance [Member]
Effects of Reinsurance [Line Items]
Incurred losses and loss adjustment expenses, prior accident years' events	825	$ 791
NICO [Member] \| AIG [Member] \| Retroactive Reinsurance [Member]
Effects of Reinsurance [Line Items]
Unpaid losses and loss adjustment expenses under retroactive reinsurance contracts	18,200		18,200
Deferred charges under retroactive reinsurance contracts	$ 6,400		$ 6,900

Unpaid losses and loss adjustment expenses for retroactive reinsurance contracts.

No definition available.

Name:	brka_LiabilityForUnpaidClaimsAndClaimAdjustmentExpenseRetroactiveReinsuranceContracts
Namespace Prefix:	brka_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The remaining unamortized excess of the estimated ultimate liability for claims and claim costs over the premiums earned with respect to retroactive property/casualty reinsurance contracts.

+ References

No definition available.

+ Details

Name:	brka_UnamortizedDeferredChargesOnReinsuranceAssumed
Namespace Prefix:	brka_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_EffectsOfReinsuranceLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

+ Details

Name:	us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=brka_RetroactiveReinsuranceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=brka_NationalIndemnityCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=brka_AmericanInternationalGroupIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: