Segment Information - Revenues (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Earned premiums and fee income	$ 4,724	$ 4,331	$ 13,470	$ 12,866
Net investment income	490	444	1,448	1,323
Realized Investment Gains (Losses)	89	38	332	60
Other revenues	44	29	129	73
Total revenues	5,347	4,842	15,379	14,322
Corporate
Segment Reporting Information [Line Items]
Earned premiums and fee income	18	15	43	21
Property and Casualty, Commercial Insurance [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	2,258	1,794	6,040	5,267
Property and Casualty, Commercial Insurance [Member] | Workers’ compensation
Segment Reporting Information [Line Items]
Earned premiums and fee income	825	845	2,480	2,495
Property and Casualty, Commercial Insurance [Member] | Liability
Segment Reporting Information [Line Items]
Earned premiums and fee income	330	170	731	480
Property and Casualty, Commercial Insurance [Member] | Marine
Segment Reporting Information [Line Items]
Earned premiums and fee income	59	0	86	0
Property and Casualty, Commercial Insurance [Member] | Package business
Segment Reporting Information [Line Items]
Earned premiums and fee income	376	343	1,092	1,013
Property and Casualty, Commercial Insurance [Member] | Property
Segment Reporting Information [Line Items]
Earned premiums and fee income	198	154	529	456
Property and Casualty, Commercial Insurance [Member] | Professional liability
Segment Reporting Information [Line Items]
Earned premiums and fee income	137	65	304	190
Property and Casualty, Commercial Insurance [Member] | Bond
Segment Reporting Information [Line Items]
Earned premiums and fee income	67	60	192	179
Property and Casualty, Commercial Insurance [Member] | Assumed Reinsurance
Segment Reporting Information [Line Items]
Earned premiums and fee income	75	0	104	0
Property and Casualty, Commercial Insurance [Member] | Automobile
Segment Reporting Information [Line Items]
Earned premiums and fee income	191	157	522	454
Property and Casualty, Personal Insurance [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	812	859	2,431	2,594
Property and Casualty, Personal Insurance [Member] | AARP Members [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	729	758	2,200	2,300
Property and Casualty, Personal Insurance [Member] | Property
Segment Reporting Information [Line Items]
Earned premiums and fee income	248	261	741	785
Property and Casualty, Personal Insurance [Member] | Automobile
Segment Reporting Information [Line Items]
Earned premiums and fee income	564	598	1,690	1,809
Group Insurance Policy [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	1,382	1,396	4,213	4,198
Group Insurance Policy [Member] | Group disability
Segment Reporting Information [Line Items]
Earned premiums and fee income	697	684	2,124	2,051
Group Insurance Policy [Member] | Group life
Segment Reporting Information [Line Items]
Earned premiums and fee income	621	652	1,902	1,968
Group Insurance Policy [Member] | Other
Segment Reporting Information [Line Items]
Earned premiums and fee income	64	60	187	179
Mutual Fund [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	254	267	743	786
Mutual Fund [Member] | Third party retail customers [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	231	242	674	710
Mutual Fund [Member] | Talcott Resolution [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	$ 23	$ 25	$ 69	$ 76