Segment Information - Non-insurance Revenue from Customers (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Segment Reporting [Line Items]
Insurance Commissions and Fees	$ 86	$ 86	$ 173	$ 172
Other Income	34	30	61	52
Non-insurance revenues with customers	111	110	220	216
Operating Segments [Member]
Segment Reporting [Line Items]
Other Income	26	25	48	46
Operating Segments [Member] \| P&C Business Insurance
Segment Reporting [Line Items]
Other Income	1	1	1	2
Operating Segments [Member] \| P&C Personal Insurance
Segment Reporting [Line Items]
Other Income	25	24	47	44
Operating Segments [Member] \| Employee Benefits [Member]
Segment Reporting [Line Items]
Other Income	0	0	0	0
Operating Segments [Member] \| Installment billing fees [Member] \| P&C Business Insurance
Segment Reporting [Line Items]
Insurance Commissions and Fees	12	11	24	22
Operating Segments [Member] \| Installment billing fees [Member] \| P&C Personal Insurance
Segment Reporting [Line Items]
Insurance Commissions and Fees	7	8	15	16
Operating Segments [Member] \| Insurance servicing revenues \| P&C Personal Insurance
Segment Reporting [Line Items]
Other Income	25	24	47	44
Operating Segments [Member] \| Administrative services \| Employee Benefits [Member]
Segment Reporting [Line Items]
Insurance Commissions and Fees	56	57	113	113
Segment Reporting, Reconciling Item, Corporate Nonsegment [Member]
Segment Reporting [Line Items]
Other Income	8	5	13	6
Segment Reporting, Reconciling Item, Corporate Nonsegment [Member] \| Investment management and other fees
Segment Reporting [Line Items]
Insurance Commissions and Fees	$ 11	$ 10	$ 21	$ 21