Segment Information - Non-insurance Revenue from Customers (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Segment Reporting [Line Items]
Insurance Commissions and Fees	$ 87	$ 84	$ 260	$ 256
Other Income	48	31	109	83
Non-insurance revenues with customers	111	109	331	325
Operating Segments [Member]
Segment Reporting [Line Items]
Other Income	24	25	72	71
Operating Segments [Member] \| P&C Business Insurance
Segment Reporting [Line Items]
Other Income	0	0	1	2
Operating Segments [Member] \| P&C Personal Insurance
Segment Reporting [Line Items]
Other Income	24	25	71	69
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	36	33
Operating Segments [Member] \| Installment billing fees [Member] \| P&C Personal Insurance
Segment Reporting [Line Items]
Insurance Commissions and Fees	7	8	22	24
Operating Segments [Member] \| Insurance servicing revenues \| P&C Personal Insurance
Segment Reporting [Line Items]
Other Income	24	25	71	69
Operating Segments [Member] \| Administrative services \| Employee Benefits [Member]
Segment Reporting [Line Items]
Insurance Commissions and Fees	57	55	170	168
Segment Reporting, Reconciling Item, Corporate Nonsegment [Member]
Segment Reporting [Line Items]
Other Income	24	6	37	12
Segment Reporting, Reconciling Item, Corporate Nonsegment [Member] \| Investment management and other fees
Segment Reporting [Line Items]
Insurance Commissions and Fees	$ 11	$ 10	$ 32	$ 31