Schedule III Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Supplementary Insurance Information [Abstract]
Deferred Policy Acquisition Costs	$ 785	$ 670
Unpaid Losses and Loss Adjustment Expenses	36,517	33,029
Reserve for Future Policy Benefits	635	642
Unearned Premiums	6,635	5,282
Other Policyholder Funds and Benefits Payable	755	767
Earned Premiums, Fee Income and Other	18,394	17,287	$ 15,394
Net Investment Income	1,951	1,780	1,603
Benefits, Losses and Loss Adjustment Expenses	11,472	11,165	10,174
Amortization of Deferred Policy Acquisition Costs [1]	1,622	1,384	1,372
Insurance Operating Costs and Other Expenses	5,086	4,653	4,893
Net Written Premiums	11,595	10,408	10,517
Group Insurance Policy [Member]
Schedule of Supplementary Insurance Information [Abstract]
Unpaid Losses and Loss Adjustment Expenses	8,256	8,445	8,512	$ 5,772
Operating Segments | Property and Casualty, Commercial Insurance [Member]
Schedule of Supplementary Insurance Information [Abstract]
Deferred Policy Acquisition Costs	615	495
Unpaid Losses and Loss Adjustment Expenses	23,363	19,455
Reserve for Future Policy Benefits	0	0
Unearned Premiums	5,015	3,589
Other Policyholder Funds and Benefits Payable	0	0
Earned Premiums, Fee Income and Other	8,326	7,081	6,902
Net Investment Income	1,129	997	949
Benefits, Losses and Loss Adjustment Expenses	5,192	4,112	4,322
Amortization of Deferred Policy Acquisition Costs [1]	1,296	1,048	1,009
Insurance Operating Costs and Other Expenses	1,776	1,396	1,381
Net Written Premiums	8,452	7,136	6,956
Operating Segments | Property and Casualty, Personal Insurance [Member]
Schedule of Supplementary Insurance Information [Abstract]
Deferred Policy Acquisition Costs	111	117
Unpaid Losses and Loss Adjustment Expenses	2,201	2,456
Reserve for Future Policy Benefits	0	0
Unearned Premiums	1,578	1,643
Other Policyholder Funds and Benefits Payable	0	0
Earned Premiums, Fee Income and Other	3,318	3,523	3,819
Net Investment Income	179	155	141
Benefits, Losses and Loss Adjustment Expenses	2,185	2,763	3,000
Amortization of Deferred Policy Acquisition Costs [1]	259	275	309
Insurance Operating Costs and Other Expenses	702	684	649
Net Written Premiums	3,131	3,276	3,561
Operating Segments | Property & Casualty Other Operations
Schedule of Supplementary Insurance Information [Abstract]
Deferred Policy Acquisition Costs	0	0
Unpaid Losses and Loss Adjustment Expenses	2,697	2,673
Reserve for Future Policy Benefits	0	0
Unearned Premiums	3	7
Other Policyholder Funds and Benefits Payable	0	0
Earned Premiums, Fee Income and Other	2	0	0
Net Investment Income	84	90	106
Benefits, Losses and Loss Adjustment Expenses	21	65	18
Amortization of Deferred Policy Acquisition Costs [1]	0	0	0
Insurance Operating Costs and Other Expenses	12	13	9
Net Written Premiums	0	(4)	0
Operating Segments | Group Insurance Policy [Member]
Schedule of Supplementary Insurance Information [Abstract]
Deferred Policy Acquisition Costs	51	52
Unpaid Losses and Loss Adjustment Expenses	8,256	8,445
Reserve for Future Policy Benefits	411	427
Unearned Premiums	39	43
Other Policyholder Funds and Benefits Payable	459	455
Earned Premiums, Fee Income and Other	5,603	5,598	3,677
Net Investment Income	486	474	381
Benefits, Losses and Loss Adjustment Expenses	4,055	4,214	2,803
Amortization of Deferred Policy Acquisition Costs [1]	54	45	33
Insurance Operating Costs and Other Expenses	1,352	1,342	924
Net Written Premiums	0	0	0
Operating Segments | Mutual Fund [Member]
Schedule of Supplementary Insurance Information [Abstract]
Deferred Policy Acquisition Costs	8	6
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	999	1,032	992
Net Investment Income	7	5	3
Benefits, Losses and Loss Adjustment Expenses	0	0	0
Amortization of Deferred Policy Acquisition Costs [1]	12	16	21
Insurance Operating Costs and Other Expenses	813	831	805
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	224	215
Unearned Premiums	0	0
Other Policyholder Funds and Benefits Payable	296	312
Earned Premiums, Fee Income and Other	146	53	4
Net Investment Income	66	59	23
Benefits, Losses and Loss Adjustment Expenses	19	11	31
Amortization of Deferred Policy Acquisition Costs [1]	1	0	0
Insurance Operating Costs and Other Expenses	431	387	1,125
Net Written Premiums	$ 12	$ 0	$ 0