Pension Plans, Retirement Benefits and Savings Plans (Details) - Fair Value - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plans [Member]
Defined benefit plan disclosure
Fair value of plan assets	$ 4,385	$ 3,874	$ 4,070
Pension Plans [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	3,525	2,871
Pension Plans [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	858	1,002
Pension Plans [Member] | Level 3 [Member]
Defined benefit plan disclosure
Fair value of plan assets	2	1
Pension Plans [Member] | Fixed maturities [Member]
Defined benefit plan disclosure
Fair value of plan assets	778	772
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	778	772
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	3	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	3	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	27
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	27
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	30	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	30	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	715	712
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	715	712
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	2,454	2,048
Pension Plans [Member] | Total mutual funds [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	2,444	2,039
Pension Plans [Member] | Total mutual funds [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	10	9
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	1,585	1,288
Pension Plans [Member] | Equity mutual funds [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	1,578	1,282
Pension Plans [Member] | Equity mutual funds [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	7	6
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	869	760
Pension Plans [Member] | Bond mutual funds [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	866	757
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	1,018	783
Pension Plans [Member] | Equity securities [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	1,017	783
Pension Plans [Member] | Equity securities [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	1	0
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	2	1
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	2	1
Pension Plans [Member] | Cash and short-term securities [Member]
Defined benefit plan disclosure
Fair value of plan assets	133	270
Pension Plans [Member] | Cash and short-term securities [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	64	49
Pension Plans [Member] | Cash and short-term securities [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	69	221
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	20	30
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	20	30
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	27	0
Pension Plans [Member] | Money market mutual funds [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	27	0
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	86	240
Pension Plans [Member] | Cash and short-term securities, other [Member] | Level 1 [Member]
Defined benefit plan disclosure
Fair value of plan assets	17	19
Pension Plans [Member] | Cash and short-term securities, other [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	69	221
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	12	12	$ 13
Postretirement Benefit Plans [Member] | Level 2 [Member]
Defined benefit plan disclosure
Fair value of plan assets	$ 12	$ 12