Schedule III Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Supplementary Insurance Information [Abstract]
Deferred Policy Acquisition Costs	$ 650	$ 645
Unpaid Losses and Loss Adjustment Expenses	32,287	28,317
Reserve for Future Policy Benefits	713	322
Unearned Premiums	5,322	5,392
Other Policyholder Funds and Benefits Payable	816	602
Earned Premiums, Fee Income and Other	15,206	14,640	$ 14,448
Net Investment Income (Loss)	1,603	1,577	1,561
Benefits, Losses and Loss Adjustment Expenses	10,174	9,961	9,324
Amortization of Deferred Policy Acquisition Costs	1,372	1,377	1,364
Insurance Operating Costs and Other Expenses	4,705	4,322	3,831
Net Written Premiums	10,517	10,568	10,578
Operating Segments | Commercial Lines
Schedule of Supplementary Insurance Information [Abstract]
Deferred Policy Acquisition Costs	467	448
Unpaid Losses and Loss Adjustment Expenses	18,893	17,950
Reserve for Future Policy Benefits	0	0
Unearned Premiums	3,504	3,441
Other Policyholder Funds and Benefits Payable	0	0
Earned Premiums, Fee Income and Other	6,902	6,690	6,551
Net Investment Income (Loss)	949	917	910
Benefits, Losses and Loss Adjustment Expenses	4,322	3,994	3,886
Amortization of Deferred Policy Acquisition Costs	1,009	973	951
Insurance Operating Costs and Other Expenses	1,381	1,244	1,232
Net Written Premiums	6,956	6,732	6,625
Operating Segments | Personal Lines
Schedule of Supplementary Insurance Information [Abstract]
Deferred Policy Acquisition Costs	127	143
Unpaid Losses and Loss Adjustment Expenses	2,294	2,094
Reserve for Future Policy Benefits	0	0
Unearned Premiums	1,768	1,898
Other Policyholder Funds and Benefits Payable	0	0
Earned Premiums, Fee Income and Other	3,819	4,023	3,997
Net Investment Income (Loss)	141	135	128
Benefits, Losses and Loss Adjustment Expenses	3,000	3,175	2,768
Amortization of Deferred Policy Acquisition Costs	309	348	359
Insurance Operating Costs and Other Expenses	649	669	714
Net Written Premiums	3,561	3,837	3,918
Operating Segments | Property & Casualty Other Operations
Schedule of Supplementary Insurance Information [Abstract]
Deferred Policy Acquisition Costs	0	0
Unpaid Losses and Loss Adjustment Expenses	2,588	2,501
Reserve for Future Policy Benefits	0	0
Unearned Premiums	10	11
Other Policyholder Funds and Benefits Payable	0	0
Earned Premiums, Fee Income and Other	0	0	32
Net Investment Income (Loss)	106	127	133
Benefits, Losses and Loss Adjustment Expenses	18	278	243
Amortization of Deferred Policy Acquisition Costs	0	0	0
Insurance Operating Costs and Other Expenses	9	663	25
Net Written Premiums	0	(1)	35
Operating Segments | Group Benefits
Schedule of Supplementary Insurance Information [Abstract]
Deferred Policy Acquisition Costs	47	42
Unpaid Losses and Loss Adjustment Expenses	8,512	5,772
Reserve for Future Policy Benefits	441	82
Unearned Premiums	40	42
Other Policyholder Funds and Benefits Payable	492	265
Earned Premiums, Fee Income and Other	3,677	3,223	3,136
Net Investment Income (Loss)	381	366	371
Benefits, Losses and Loss Adjustment Expenses	2,803	2,514	2,427
Amortization of Deferred Policy Acquisition Costs	33	31	31
Insurance Operating Costs and Other Expenses	924	776	788
Net Written Premiums	0	0	0
Operating Segments | Mutual Funds
Schedule of Supplementary Insurance Information [Abstract]
Deferred Policy Acquisition Costs	9	12
Unpaid Losses and Loss Adjustment Expenses	0	0
Reserve for Future Policy Benefits	0	0
Unearned Premiums	0	0
Other Policyholder Funds and Benefits Payable	0	0
Earned Premiums, Fee Income and Other	804	701	723
Net Investment Income (Loss)	3	1	1
Benefits, Losses and Loss Adjustment Expenses	0	0	0
Amortization of Deferred Policy Acquisition Costs	21	24	22
Insurance Operating Costs and Other Expenses	617	557	568
Net Written Premiums	0	0	0
Corporate
Schedule of Supplementary Insurance Information [Abstract]
Deferred Policy Acquisition Costs	0	0
Unpaid Losses and Loss Adjustment Expenses	0	0
Reserve for Future Policy Benefits	272	240
Unearned Premiums	0	0
Other Policyholder Funds and Benefits Payable	324	337
Earned Premiums, Fee Income and Other	4	3	9
Net Investment Income (Loss)	23	31	18
Benefits, Losses and Loss Adjustment Expenses	31	0	0
Amortization of Deferred Policy Acquisition Costs	0	1	1
Insurance Operating Costs and Other Expenses	1,125	413	504
Net Written Premiums	$ 0	$ 0	$ 0