Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
Total revenues	$ 13,299	$ 11,879	[1]	$ 11,908	[1]
Total operating revenues	13,398	12,078		11,788
Net investment gains (losses)	(99)	(199)	[1]	120	[1]
Operating Segments | Specialty
Total revenues	5,490	5,209		5,004
Operating Segments | Commercial
Total revenues	5,221	4,441		4,199
Operating Segments | International
Total revenues	1,279	1,134		1,114
Operating Segments | Life & Group
Total revenues	1,346	1,276		1,457
Operating Segments | Corporate & Other
Total revenues	73	24		21
Eliminations
Total revenues	(11)	(6)		(7)
Management & Professional Liability | Operating Segments | Specialty
Total revenues	2,931	2,771		2,776
Surety | Operating Segments | Specialty
Total revenues	731	652		604
Warranty & Alternative Risks | Operating Segments | Specialty
Total revenues	1,828	1,786		1,624
Middle Market | Operating Segments | Commercial
Total revenues	1,696	1,532		1,508
Construction | Operating Segments | Commercial
Total revenues	1,678	1,421		1,322
Small Business | Operating Segments | Commercial
Total revenues	631	581		558
Other Commercial | Operating Segments | Commercial
Total revenues	1,216	907		811
Canada | Operating Segments | International
Total revenues	383	366		344
Europe | Operating Segments | International
Total revenues	532	466		473
Hardy | Operating Segments | International
Total revenues	$ 364	$ 302		$ 297

[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 Consolidated Financial Statements for additional information.