Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Total revenues	$ 3,152	$ 2,885	[1]
Total operating revenues	3,187	2,896
Net investment gains (losses)	(35)	(11)	[1]
Operating Segments | Specialty
Total revenues	1,333	1,258
Operating Segments | Commercial
Total revenues	1,202	1,030
Operating Segments | International
Total revenues	313	277
Operating Segments | Life & Group
Total revenues	329	331
Operating Segments | Corporate & Other
Total revenues	13	2
Eliminations
Total revenues	(3)	(2)
Management & Professional Liability | Operating Segments | Specialty
Total revenues	705	673
Surety | Operating Segments | Specialty
Total revenues	168	148
Warranty & Alternative Risks | Operating Segments | Specialty
Total revenues	460	437
Middle Market | Operating Segments | Commercial
Total revenues	398	362
Construction | Operating Segments | Commercial
Total revenues	385	324
Small Business | Operating Segments | Commercial
Total revenues	150	138
Other Commercial | Operating Segments | Commercial
Total revenues	269	206
Canada | Operating Segments | International
Total revenues	93	88
Europe | Operating Segments | International
Total revenues	127	120
Hardy | Operating Segments | International
Total revenues	$ 93	$ 69

[1] As of January 1, 2023, the Company adopted ASU 2018-12, Financial Services-Insurance (Topic 944): Targeted Improvements to the Accounting for Long-Duration Contracts (ASU 2018-12) using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been adjusted to reflect application of the new guidance. See Note A to the Condensed Consolidated Financial Statements for additional information.