Segment Information - Revenues (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Earned premiums and fee income	$ 4,331	$ 3,738	$ 12,866	$ 11,195
Net investment income	444	404	1,323	1,209
Realized Investment Gains (Losses)	38	26	60	105
Other revenues	29	24	73	66
Total revenues	4,842	4,192	14,322	12,575
Property and Casualty, Commercial Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	1,794	1,732	5,267	5,159
Property and Casualty, Commercial Insurance Product Line [Member] | Workers’ compensation
Segment Reporting Information [Line Items]
Earned premiums and fee income	845	828	2,495	2,461
Property and Casualty, Commercial Insurance Product Line [Member] | Liability
Segment Reporting Information [Line Items]
Earned premiums and fee income	170	150	480	450
Property and Casualty, Commercial Insurance Product Line [Member] | Package business
Segment Reporting Information [Line Items]
Earned premiums and fee income	343	325	1,013	965
Property and Casualty, Commercial Insurance Product Line [Member] | Automobile
Segment Reporting Information [Line Items]
Earned premiums and fee income	157	155	454	477
Property and Casualty, Commercial Insurance Product Line [Member] | Professional liability
Segment Reporting Information [Line Items]
Earned premiums and fee income	65	63	190	183
Property and Casualty, Commercial Insurance Product Line [Member] | Bond
Segment Reporting Information [Line Items]
Earned premiums and fee income	60	59	179	172
Property and Casualty, Commercial Insurance Product Line [Member] | Property
Segment Reporting Information [Line Items]
Earned premiums and fee income	154	152	456	451
Property and Casualty, Personal Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	859	932	2,594	2,818
Property and Casualty, Personal Insurance Product Line [Member] | AARP Members [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	758	801	2,300	2,400
Property and Casualty, Personal Insurance Product Line [Member] | Automobile
Segment Reporting Information [Line Items]
Earned premiums and fee income	598	653	1,809	1,975
Property and Casualty, Personal Insurance Product Line [Member] | Property
Segment Reporting Information [Line Items]
Earned premiums and fee income	261	279	785	843
Group Insurance Policies [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	1,396	822	4,198	2,481
Group Insurance Policies [Member] | Group disability
Segment Reporting Information [Line Items]
Earned premiums and fee income	684	386	2,051	1,146
Group Insurance Policies [Member] | Group life
Segment Reporting Information [Line Items]
Earned premiums and fee income	652	383	1,968	1,176
Group Insurance Policies [Member] | Other
Segment Reporting Information [Line Items]
Earned premiums and fee income	60	53	179	159
Mutual Fund [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	267	251	786	735
Mutual Fund [Member] | Third party retail customers [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	242	225	710	657
Mutual Fund [Member] | Talcott Resolution [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	25	26	76	78
Corporate
Segment Reporting Information [Line Items]
Earned premiums and fee income	$ 15	1	$ 21	2
Operating Expense [Member]
Segment Reporting Information [Line Items]
Distribution Costs		$ 48		$ 140