Claim, Claim Adjustment Expense and Future Policy Benefit Reserves, Workers' Compensation (Details) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($) Claim	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2012	$ 3,830	$ 4,216	$ 4,584
Total net liability for unpaid claim and claim adjustment expenses	19,205	18,701	17,885	$ 17,965
Property and casualty
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total net liability for unpaid claim and claim adjustment expenses	15,270	14,195
Workers' Compensation
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	4,575
IBNR	1,325
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	2,703
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	1,872
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2012	1,941
Other	(14)
Liability for unallocated claim adjustment expenses for accident years presented	51
Total net liability for unpaid claim and claim adjustment expenses	3,850
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total net development for the accident years presented above	(49)	(85)	(77)
Adjustment for development on a discounted basis	2	2	3
Total net development for accident years prior to 2012	(35)	(13)	38
Total unallocated claim adjustment expense development	0	0	23
Total	(82)	(96)	(13)
Workers' Compensation | Short-duration Insurance Contracts, Accident Year 2012
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	664	663	668	671	$ 673	$ 678	$ 669	$ 659	$ 627	$ 601
IBNR	$ 66
Cumulative number of claims | Claim	42,804
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 541	538	536	524	509	470	416	342	232	87
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	1	(5)	(3)	(2)	(5)	9	10	32	26
IBNR, Total	63
Workers' Compensation | Short-duration Insurance Contracts, Accident Year 2013
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	548	552	561	582	593	618	592	572	537
IBNR	$ 91
Cumulative number of claims | Claim	38,867
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 417	414	411	419	417	370	300	213	80
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(4)	(9)	(21)	(11)	(25)	26	20	35
IBNR, Total	11
Workers' Compensation | Short-duration Insurance Contracts, Accident Year 2014
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	448	439	446	450	452	479	480	467
IBNR	$ 105
Cumulative number of claims | Claim	33,502
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 306	297	290	282	258	215	159	61
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	9	(7)	(4)	(2)	(27)	(1)	13
IBNR, Total	(19)
Workers' Compensation | Short-duration Insurance Contracts, Accident Year 2015
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	372	382	394	408	406	431	422
IBNR	$ 101
Cumulative number of claims | Claim	31,894
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 251	243	231	212	180	131	51
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(10)	(12)	(14)	2	(25)	9
IBNR, Total	(50)
Workers' Compensation | Short-duration Insurance Contracts, Accident Year 2016
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	355	366	382	396	405	426
IBNR	$ 104
Cumulative number of claims | Claim	31,981
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 227	219	198	169	129	53
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(11)	(16)	(14)	(9)	(21)
IBNR, Total	(71)
Workers' Compensation | Short-duration Insurance Contracts, Accident Year 2017
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	402	400	421	432	440
IBNR	$ 92
Cumulative number of claims | Claim	33,121
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 265	243	207	151	63
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	2	(21)	(11)	(8)
IBNR, Total	(38)
Workers' Compensation | Short-duration Insurance Contracts, Accident Year 2018
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	415	428	440	450
IBNR	$ 104
Cumulative number of claims | Claim	34,851
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 259	229	163	68
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(13)	(12)	(10)
IBNR, Total	(35)
Workers' Compensation | Short-Duration Insurance Contract, Accident Year 2019
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	437	449	452
IBNR	$ 134
Cumulative number of claims | Claim	34,248
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 223	169	71
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(12)	(3)
IBNR, Total	(15)
Workers' Compensation | Short-Duration Insurance Contract, Accident Year 2020
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	466	477
IBNR	$ 228
Cumulative number of claims | Claim	29,188
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 147	65
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(11)
IBNR, Total	(11)
Workers' Compensation | Short-Duration Insurance Contract, Accident Year 2021
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	468
IBNR	$ 300
Cumulative number of claims | Claim	25,711
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 67
General Liability
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	6,670
IBNR	2,006
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	3,929
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	2,741
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2012	119
Liability for unallocated claim adjustment expenses for accident years presented	51
Total net liability for unpaid claim and claim adjustment expenses	2,911
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total net development for the accident years presented above	14	16	21
Total net development for accident years prior to 2012	(1)	(1)	(4)
Total unallocated claim adjustment expense development	2	0	(1)
Total	15	15	16
General Liability | Short-duration Insurance Contracts, Accident Year 2012
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	632	632	630	635	635	619	636	639	611	587
IBNR	$ 20
Cumulative number of claims | Claim	35,313
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 602	597	579	559	510	454	374	247	132	$ 28
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	0	2	(5)	0	16	(17)	(3)	28	24
IBNR, Total	45
General Liability | Short-duration Insurance Contracts, Accident Year 2013
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	624	623	620	623	613	655	650	655	650
IBNR	$ 24
Cumulative number of claims | Claim	33,706
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 582	572	551	510	450	352	240	128	$ 31
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	1	3	(3)	10	(42)	5	(5)	5
IBNR, Total	(26)
General Liability | Short-duration Insurance Contracts, Accident Year 2014
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	659	659	658	635	631	654	658	653
IBNR	$ 28
Cumulative number of claims | Claim	28,064
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 607	569	547	481	376	247	119	$ 31
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	0	1	23	4	(23)	(4)	5
IBNR, Total	6
General Liability | Short-duration Insurance Contracts, Accident Year 2015
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	617	602	600	589	574	576	581
IBNR	$ 42
Cumulative number of claims | Claim	24,118
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 530	501	446	357	230	110	$ 19
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	15	2	11	15	(2)	(5)
IBNR, Total	36
General Liability | Short-duration Insurance Contracts, Accident Year 2016
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	683	673	671	667	659	623
IBNR	$ 68
Cumulative number of claims | Claim	24,511
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 524	481	407	279	163	$ 32
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	10	2	4	8	36
IBNR, Total	60
General Liability | Short-duration Insurance Contracts, Accident Year 2017
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	630	634	632	632	632
IBNR	$ 67
Cumulative number of claims | Claim	22,195
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 471	399	250	118	$ 23
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(4)	2	0	0
IBNR, Total	(2)
General Liability | Short-duration Insurance Contracts, Accident Year 2018
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	639	646	644	653
IBNR	$ 205
Cumulative number of claims | Claim	19,917
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 307	228	107	$ 33
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(7)	2	(9)
IBNR, Total	(14)
General Liability | Short-Duration Insurance Contract, Accident Year 2019
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	682	682	680
IBNR	$ 330
Cumulative number of claims | Claim	18,602
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 181	98	$ 25
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	0	2
IBNR, Total	2
General Liability | Short-Duration Insurance Contract, Accident Year 2020
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	722	723
IBNR	$ 516
Cumulative number of claims | Claim	13,028
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 99	$ 23
Net Strengthening (Releases) of Prior Accident Year Reserves [Abstract]
Total	(1)
IBNR, Total	(1)
General Liability | Short-Duration Insurance Contract, Accident Year 2021
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	782
IBNR	$ 706
Cumulative number of claims | Claim	9,759
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses [Abstract]
Total	$ 26