Pension Plans, Retirement Benefits and Savings Plans (Details) - USD ($)
$ in Millions |
3 Months Ended |
Mar. 31, 2020 |
Mar. 31, 2019 |
| Pension Plans [Member] |
|
|
| Defined benefit plan disclosure |
|
|
| Service cost |
$ 33
|
$ 30
|
| Interest cost on benefit obligation |
29
|
35
|
| Expected return on plan assets |
(69)
|
(69)
|
| Net periodic benefit cost, amortization of unrecognized prior service benefit |
0
|
0
|
| Net periodic benefit cost, amortization of unrecognized net actuarial loss |
23
|
14
|
| Total non-service cost (benefit) |
(17)
|
(20)
|
| Net periodic benefit cost |
16
|
10
|
| Pension Plans [Member] | Claims and claim adjustment expenses [Member] |
|
|
| Defined benefit plan disclosure |
|
|
| Service cost |
14
|
12
|
| Total non-service cost (benefit) |
(7)
|
(8)
|
| Pension Plans [Member] | General and administrative expenses [Member] |
|
|
| Defined benefit plan disclosure |
|
|
| Service cost |
19
|
18
|
| Total non-service cost (benefit) |
(10)
|
(12)
|
| Postretirement Benefit Plans [Member] |
|
|
| Defined benefit plan disclosure |
|
|
| Service cost |
0
|
0
|
| Interest cost on benefit obligation |
1
|
2
|
| Expected return on plan assets |
0
|
0
|
| Net periodic benefit cost, amortization of unrecognized prior service benefit |
(2)
|
(1)
|
| Net periodic benefit cost, amortization of unrecognized net actuarial loss |
0
|
0
|
| Total non-service cost (benefit) |
(1)
|
1
|
| Net periodic benefit cost |
(1)
|
1
|
| Postretirement Benefit Plans [Member] | Claims and claim adjustment expenses [Member] |
|
|
| Defined benefit plan disclosure |
|
|
| Service cost |
0
|
0
|
| Total non-service cost (benefit) |
0
|
0
|
| Postretirement Benefit Plans [Member] | General and administrative expenses [Member] |
|
|
| Defined benefit plan disclosure |
|
|
| Service cost |
0
|
0
|
| Total non-service cost (benefit) |
$ (1)
|
$ 1
|