Benefit Plans, Other Postretirement Benefit Plan Assets Measured at Fair Value on Recurring Basis (Details) - Other Postretirement Benefits [Member] - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Other Postretirement Benefits Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets	$ 96	$ 90	$ 85
Corporate and Other Bonds [Member]
Other Postretirement Benefits Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets	24	22
States, Municipalities and Political Subdivisions [Member]
Other Postretirement Benefits Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets	14	16
Asset-Backed [Member]
Other Postretirement Benefits Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets	33	31
Fixed Maturity Securities [Member]
Other Postretirement Benefits Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets	71	69
Short Term Investments [Member]
Other Postretirement Benefits Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets	5	3
Fixed Income Mutual Funds [Member]
Other Postretirement Benefits Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets	20	18
Level 1 [Member]
Other Postretirement Benefits Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets	25	21
Level 1 [Member] | Fixed Maturity Securities [Member]
Other Postretirement Benefits Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets	0	0
Level 1 [Member] | Short Term Investments [Member]
Other Postretirement Benefits Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets	5	3
Level 1 [Member] | Fixed Income Mutual Funds [Member]
Other Postretirement Benefits Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets	20	18
Level 2 [Member]
Other Postretirement Benefits Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets	71	69
Level 2 [Member] | Corporate and Other Bonds [Member]
Other Postretirement Benefits Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets	24	22
Level 2 [Member] | States, Municipalities and Political Subdivisions [Member]
Other Postretirement Benefits Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets	14	16
Level 2 [Member] | Asset-Backed [Member]
Other Postretirement Benefits Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets	33	31
Level 2 [Member] | Fixed Maturity Securities [Member]
Other Postretirement Benefits Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets	71	69
Level 3 [Member]
Other Postretirement Benefits Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets	0	0
Level 3 [Member] | Fixed Maturity Securities [Member]
Other Postretirement Benefits Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets	$ 0	$ 0