Segment Information (Details) - Amortization and Depreciation - Reportable segments [Member] - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment reporting information
Amortization and depreciation	$ 7,682	$ 6,942	$ 6,334
Business Insurance [Member]
Segment reporting information
Amortization and depreciation	4,014	3,640	3,344
Bond & Specialty Insurance [Member]
Segment reporting information
Amortization and depreciation	842	744	697
Personal Insurance [Member]
Segment reporting information
Amortization and depreciation	$ 2,826	$ 2,558	$ 2,293

The aggregate net amount of depreciation, amortization, and accretion recognized during an accounting period. As a noncash item, the net amount is added back to net income when calculating cash provided by or used in operations using the indirect method.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
-SubTopic 10
-Topic 230
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-28

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