Pension Plans, Retirement Benefits and Savings Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Pension Plans [Member]
Defined benefit plan disclosure
Service cost	$ 27	$ 36	$ 81	$ 109
Interest cost on benefit obligation	44	25	132	76
Expected return on plan assets	(77)	(74)	(233)	(222)
Net periodic benefit cost (benefit), amortization of unrecognized prior service benefit	(1)	0	(1)	0
Net periodic benefit cost (benefit), amortization of unrecognized net actuarial (gain) loss	0	12	0	37
Total non-service cost (benefit)	(34)	(37)	(102)	(109)
Net periodic benefit cost (benefit)	(7)	(1)	(21)	0
Pension Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	11	14	33	44
Total non-service cost (benefit)	(14)	(15)	(41)	(44)
Pension Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	16	22	48	65
Total non-service cost (benefit)	(20)	(22)	(61)	(65)
Postretirement Benefit Plans [Member]
Defined benefit plan disclosure
Service cost	0	0	0	0
Interest cost on benefit obligation	2	0	4	2
Expected return on plan assets	0	0	0	0
Net periodic benefit cost (benefit), amortization of unrecognized prior service benefit	(1)	0	(3)	(2)
Net periodic benefit cost (benefit), amortization of unrecognized net actuarial (gain) loss	(3)	(1)	(7)	(3)
Total non-service cost (benefit)	(2)	(1)	(6)	(3)
Net periodic benefit cost (benefit)	(2)	(1)	(6)	(3)
Postretirement Benefit Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	0	0	0	0
Total non-service cost (benefit)	(1)	0	(3)	(1)
Postretirement Benefit Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	0	0	0	0
Total non-service cost (benefit)	$ (1)	$ (1)	$ (3)	$ (2)