Pension Plans, Retirement Benefits and Savings Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Pension Plans [Member]
Defined benefit plan disclosure
Service cost	$ 36	$ 33	$ 71	$ 66
Interest cost on benefit obligation	20	28	41	57
Expected return on plan assets	(68)	(68)	(137)	(137)
Net periodic benefit cost (benefit), amortization of unrecognized prior service benefit	0	0	0	0
Net periodic benefit cost (benefit), amortization of unrecognized net actuarial (gain) loss	27	23	54	46
Total non-service cost (benefit)	(21)	(17)	(42)	(34)
Net periodic benefit cost (benefit)	15	16	29	32
Pension Plans [Member] | Claims and claim adjustment expenses [Member]
Defined benefit plan disclosure
Service cost	15	14	29	28
Total non-service cost (benefit)	(8)	(7)	(17)	(14)
Pension Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	21	19	42	38
Total non-service cost (benefit)	(13)	(10)	(25)	(20)
Postretirement Benefit Plans [Member]
Defined benefit plan disclosure
Service cost	0	0	0	0
Interest cost on benefit obligation	0	1	1	2
Expected return on plan assets	0	0	0	0
Net periodic benefit cost (benefit), amortization of unrecognized prior service benefit	(1)	0	(2)	(2)
Net periodic benefit cost (benefit), amortization of unrecognized net actuarial (gain) loss	0	(1)	(1)	(1)
Total non-service cost (benefit)	(1)	0	(2)	(1)
Net periodic benefit cost (benefit)	(1)	0	(2)	(1)
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	(1)	0
Postretirement Benefit Plans [Member] | General and administrative expenses [Member]
Defined benefit plan disclosure
Service cost	0	0	0	0
Total non-service cost (benefit)	$ 0	$ 0	$ (1)	$ (1)