Pension Plans, Retirement Benefits and Savings Plans (Details) - Fair Value - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plans [Member]
Defined benefit plan disclosure
Fair value of plan assets	$ 4,756	$ 4,385	$ 3,874
Pension Plans [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	3,870	3,525
Pension Plans [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	885	858
Pension Plans [Member] | Level 3 [Member]
Defined benefit plan disclosure
Fair value of plan assets	1	2
Pension Plans [Member] | Fixed maturities [Member]
Defined benefit plan disclosure
Fair value of plan assets	783	778
Pension Plans [Member] | Fixed maturities [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	0
Pension Plans [Member] | Fixed maturities [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	783	778
Pension Plans [Member] | Fixed maturities [Member] | Level 3 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	0
Pension Plans [Member] | Obligations of states, municipalities and political subdivisions [Member]
Defined benefit plan disclosure
Fair value of plan assets	27	3
Pension Plans [Member] | Obligations of states, municipalities and political subdivisions [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	0
Pension Plans [Member] | Obligations of states, municipalities and political subdivisions [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	27	3
Pension Plans [Member] | Obligations of states, municipalities and political subdivisions [Member] | Level 3 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	0
Pension Plans [Member] | Debt securities issued by foreign governments [Member]
Defined benefit plan disclosure
Fair value of plan assets	30	30
Pension Plans [Member] | Debt securities issued by foreign governments [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	0
Pension Plans [Member] | Debt securities issued by foreign governments [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	30	30
Pension Plans [Member] | Debt securities issued by foreign governments [Member] | Level 3 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	0
Pension Plans [Member] | Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Defined benefit plan disclosure
Fair value of plan assets	14	30
Pension Plans [Member] | Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	0
Pension Plans [Member] | Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	14	30
Pension Plans [Member] | Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member] | Level 3 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	0
Pension Plans [Member] | All other corporate bonds [Member]
Defined benefit plan disclosure
Fair value of plan assets	712	715
Pension Plans [Member] | All other corporate bonds [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	0
Pension Plans [Member] | All other corporate bonds [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	712	715
Pension Plans [Member] | All other corporate bonds [Member] | Level 3 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	0
Pension Plans [Member] | Total mutual funds [Member]
Defined benefit plan disclosure
Fair value of plan assets	2,655	2,454
Pension Plans [Member] | Total mutual funds [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	2,645	2,444
Pension Plans [Member] | Total mutual funds [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	10	10
Pension Plans [Member] | Total mutual funds [Member] | Level 3 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	0
Pension Plans [Member] | Equity mutual funds [Member]
Defined benefit plan disclosure
Fair value of plan assets	1,706	1,585
Pension Plans [Member] | Equity mutual funds [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	1,699	1,578
Pension Plans [Member] | Equity mutual funds [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	7	7
Pension Plans [Member] | Equity mutual funds [Member] | Level 3 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	0
Pension Plans [Member] | Bond mutual funds [Member]
Defined benefit plan disclosure
Fair value of plan assets	949	869
Pension Plans [Member] | Bond mutual funds [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	946	866
Pension Plans [Member] | Bond mutual funds [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	3	3
Pension Plans [Member] | Bond mutual funds [Member] | Level 3 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	0
Pension Plans [Member] | Equity securities [Member]
Defined benefit plan disclosure
Fair value of plan assets	1,171	1,018
Pension Plans [Member] | Equity securities [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	1,171	1,017
Pension Plans [Member] | Equity securities [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	1
Pension Plans [Member] | Equity securities [Member] | Level 3 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	0
Pension Plans [Member] | Other investments [Member]
Defined benefit plan disclosure
Fair value of plan assets	1	2
Pension Plans [Member] | Other investments [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	0
Pension Plans [Member] | Other investments [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	0
Pension Plans [Member] | Other investments [Member] | Level 3 [Member]
Defined benefit plan disclosure
Fair value of plan assets	1	2
Pension Plans [Member] | Cash and short-term securities [Member]
Defined benefit plan disclosure
Fair value of plan assets	146	133
Pension Plans [Member] | Cash and short-term securities [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	54	64
Pension Plans [Member] | Cash and short-term securities [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	92	69
Pension Plans [Member] | Cash and short-term securities [Member] | Level 3 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	0
Pension Plans [Member] | Cash and short-term securities, U.S. Treasury securities [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	20
Pension Plans [Member] | Cash and short-term securities, U.S. Treasury securities [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	20
Pension Plans [Member] | Cash and short-term securities, U.S. Treasury securities [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	0
Pension Plans [Member] | Cash and short-term securities, U.S. Treasury securities [Member] | Level 3 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	0
Pension Plans [Member] | Money market mutual funds [Member]
Defined benefit plan disclosure
Fair value of plan assets	26	27
Pension Plans [Member] | Money market mutual funds [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	26	27
Pension Plans [Member] | Money market mutual funds [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	0
Pension Plans [Member] | Money market mutual funds [Member] | Level 3 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	0
Pension Plans [Member] | Cash and short-term securities, other [Member]
Defined benefit plan disclosure
Fair value of plan assets	120	86
Pension Plans [Member] | Cash and short-term securities, other [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	28	17
Pension Plans [Member] | Cash and short-term securities, other [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	92	69
Pension Plans [Member] | Cash and short-term securities, other [Member] | Level 3 [Member]
Defined benefit plan disclosure
Fair value of plan assets	0	0
Postretirement Benefit Plans [Member]
Defined benefit plan disclosure
Fair value of plan assets	11	12	$ 12
Postretirement Benefit Plans [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	$ 11	$ 12