Business Segments - Balance Sheet Information (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 4,433		$ 4,730
Insurance receivables	2,191		2,201
Deferred acquisition costs	626	$ 600	622
Goodwill	146	145	150
Insurance reserves
Claim and claim adjustment expenses	22,260	22,343	23,018
Unearned premiums	3,912	3,762	3,807
Future policy benefits	10,491	$ 10,326	10,500
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Reinsurance receivables	817		765
Insurance receivables	874		884
Deferred acquisition costs	314		309
Goodwill	117		117
Insurance reserves
Claim and claim adjustment expenses	6,224		6,325
Unearned premiums	1,941		1,861
Future policy benefits	0		0
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Reinsurance receivables	616		623
Insurance receivables	1,048		1,025
Deferred acquisition costs	225		225
Goodwill	0		0
Insurance reserves
Claim and claim adjustment expenses	8,760		9,095
Unearned premiums	1,375		1,347
Future policy benefits	0		0
Operating Segments | International
Segment Reporting Information [Line Items]
Reinsurance receivables	133		139
Insurance receivables	254		276
Deferred acquisition costs	87		88
Goodwill	29		33
Insurance reserves
Claim and claim adjustment expenses	1,343		1,395
Unearned premiums	450		464
Future policy benefits	0		0
Operating Segments | Life & Group Non-Core
Segment Reporting Information [Line Items]
Reinsurance receivables	448		496
Insurance receivables	12		14
Deferred acquisition costs	0		0
Goodwill	0		0
Insurance reserves
Claim and claim adjustment expenses	3,373		3,311
Unearned premiums	147		136
Future policy benefits	10,491		10,500
Operating Segments | Corporate & Other Non-Core
Segment Reporting Information [Line Items]
Reinsurance receivables	2,419		2,707
Insurance receivables	3		2
Deferred acquisition costs	0		0
Goodwill	0		0
Insurance reserves
Claim and claim adjustment expenses	2,560		2,892
Unearned premiums	0		0
Future policy benefits	0		0
Intersegment Eliminations
Segment Reporting Information [Line Items]
Reinsurance receivables	0		0
Insurance receivables	0		0
Deferred acquisition costs	0		0
Goodwill	0		0
Insurance reserves
Claim and claim adjustment expenses	0		0
Unearned premiums	(1)		(1)
Future policy benefits	$ 0		$ 0