Segment Information - Revenues (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Earned premiums and fee income	$ 4,250	$ 3,716
Net investment income	451	410
Realized Investment Gains (Losses)	(30)	24
Other Income	20	19
Total revenues	4,691	4,169
Operating Segments | Commercial Lines Segment [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	1,720	1,698
Operating Segments | Commercial Lines Segment [Member] | Workers' compensation
Segment Reporting Information [Line Items]
Earned premiums and fee income	818	813
Operating Segments | Commercial Lines Segment [Member] | Liability
Segment Reporting Information [Line Items]
Earned premiums and fee income	151	148
Operating Segments | Commercial Lines Segment [Member] | Package business
Segment Reporting Information [Line Items]
Earned premiums and fee income	332	314
Operating Segments | Commercial Lines Segment [Member] | Automobile
Segment Reporting Information [Line Items]
Earned premiums and fee income	149	161
Operating Segments | Commercial Lines Segment [Member] | Professional liability
Segment Reporting Information [Line Items]
Earned premiums and fee income	62	60
Operating Segments | Commercial Lines Segment [Member] | Bond
Segment Reporting Information [Line Items]
Earned premiums and fee income	58	55
Operating Segments | Commercial Lines Segment [Member] | Property
Segment Reporting Information [Line Items]
Earned premiums and fee income	150	147
Operating Segments | Personal Lines Segment [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	869	945
Operating Segments | Personal Lines Segment [Member] | AARP Members [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	758	800
Operating Segments | Personal Lines Segment [Member] | Automobile
Segment Reporting Information [Line Items]
Earned premiums and fee income	607	662
Operating Segments | Personal Lines Segment [Member] | Homeowners
Segment Reporting Information [Line Items]
Earned premiums and fee income	262	283
Operating Segments | Group Benefits Segment [Member]
Segment Reporting Information [Line Items]
Earned premiums and fee income	1,401	835
Operating Segments | Group Benefits Segment [Member] | Group disability
Segment Reporting Information [Line Items]
Earned premiums and fee income	677	381
Operating Segments | Group Benefits Segment [Member] | Group life
Segment Reporting Information [Line Items]
Earned premiums and fee income	664	399
Operating Segments | Group Benefits Segment [Member] | Other
Segment Reporting Information [Line Items]
Earned premiums and fee income	60	55
Operating Segments | Mutual Funds
Segment Reporting Information [Line Items]
Earned premiums and fee income	258	237
Operating Segments | Mutual Funds | Discontinued Operations, Held-for-sale
Segment Reporting Information [Line Items]
Earned premiums and fee income	26	26
Operating Segments | Mutual Funds | Mutual Fund
Segment Reporting Information [Line Items]
Earned premiums and fee income	232	211
Corporate
Segment Reporting Information [Line Items]
Earned premiums and fee income	$ 2	$ 1