Segment Information - Revenues (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Earned premiums and fee income	$ 4,285	$ 3,741	$ 8,535	$ 7,457
Net investment income	428	395	879	805
Realized Investment Gains (Losses)	52	55	22	79
Other revenues	24	23	44	42
Total revenues	4,789	4,214	9,480	8,383
Property and Casualty, Commercial Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	1,753	1,729	3,473	3,427
Property and Casualty, Commercial Insurance Product Line [Member] | Workers’ compensation
Segment Reporting Information [Line Items]
Earned premiums and fee income	832	820	1,650	1,633
Property and Casualty, Commercial Insurance Product Line [Member] | Liability
Segment Reporting Information [Line Items]
Earned premiums and fee income	159	152	310	300
Property and Casualty, Commercial Insurance Product Line [Member] | Package business
Segment Reporting Information [Line Items]
Earned premiums and fee income	338	326	670	640
Property and Casualty, Commercial Insurance Product Line [Member] | Automobile
Segment Reporting Information [Line Items]
Earned premiums and fee income	148	161	297	322
Property and Casualty, Commercial Insurance Product Line [Member] | Professional liability
Segment Reporting Information [Line Items]
Earned premiums and fee income	63	60	125	120
Property and Casualty, Commercial Insurance Product Line [Member] | Bond
Segment Reporting Information [Line Items]
Earned premiums and fee income	61	58	119	113
Property and Casualty, Commercial Insurance Product Line [Member] | Property
Segment Reporting Information [Line Items]
Earned premiums and fee income	152	152	302	299
Property and Casualty, Personal Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	866	941	1,735	1,886
Property and Casualty, Personal Insurance Product Line [Member] | AARP Members [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	758	800	1,500	1,600
Property and Casualty, Personal Insurance Product Line [Member] | Automobile
Segment Reporting Information [Line Items]
Earned premiums and fee income	604	660	1,211	1,322
Property and Casualty, Personal Insurance Product Line [Member] | Property
Segment Reporting Information [Line Items]
Earned premiums and fee income	262	281	524	564
Group Insurance Policies [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	1,401	824	2,802	1,659
Group Insurance Policies [Member] | Group disability
Segment Reporting Information [Line Items]
Earned premiums and fee income	690	379	1,367	760
Group Insurance Policies [Member] | Group life
Segment Reporting Information [Line Items]
Earned premiums and fee income	652	394	1,316	793
Group Insurance Policies [Member] | Other
Segment Reporting Information [Line Items]
Earned premiums and fee income	59	51	119	106
Mutual Fund [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	261	247	519	484
Mutual Fund [Member] | Talcott Resolution [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	25	26	51	52
Mutual Fund [Member] | Mutual fund and Exchange-Traded Products (ETP) [2]
Segment Reporting Information [Line Items]
Earned premiums and fee income	236	221	468	432
Corporate
Segment Reporting Information [Line Items]
Earned premiums and fee income	$ 4	0	$ 6	1
Operating Expense [Member]
Segment Reporting Information [Line Items]
Distribution Costs		$ 46		$ 92