Reserve for Property and Casualty Insurance Claims and Claims Expense - Narrative (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020
Catastrophe
Increase (decrease) in claims and claims expense
Incurred expense (adjustment)	$ 2,810	$ 2,390

Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred and costs incurred in the claim settlement process.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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

Name:	us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaims1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration