Segment Information - Amortization of Other Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Amortization of other intangible assets	$ 71	$ 71	$ 71
Operating Segments | P&C Commercial Lines
Segment Reporting Information [Line Items]
Amortization of other intangible assets	29	29	29
Operating Segments | P&C Personal Lines
Segment Reporting Information [Line Items]
Amortization of other intangible assets	2	2	2
Operating Segments | Group Benefits [Member]
Segment Reporting Information [Line Items]
Amortization of other intangible assets	$ 40	$ 40	$ 40