Benefit Plans, Pension Plan Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Limited Partnership Investments [Abstract]
Percentage of carrying value of investments employing hedge fund strategies		75.00%	80.00%
Hedge Funds, Equity Related [Member]
Limited Partnership Investments [Abstract]
Percentage of carrying value of investments in limited partnerships		69.00%
Hedge Funds, Multi-Strategy Approach Hedge Funds [Member]
Limited Partnership Investments [Abstract]
Percentage of carrying value of investments in limited partnerships		27.00%
Hedge Funds, Distressed Investments [Member]
Limited Partnership Investments [Abstract]
Percentage of carrying value of investments in limited partnerships		4.00%
Pension Benefits [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		$ 2,739	$ 2,576	$ 2,304
Pension Benefits [Member] | Level 1, 2 and 3 [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		1,926	1,700
Pension Benefits [Member] | Level 1 [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		961	754
Pension Benefits [Member] | Level 2 [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		956	936
Pension Benefits [Member] | Level 3 [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		9	10
Pension Benefits [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets	[1]	813	876
Pension Benefits [Member] | Fixed Maturity Securities [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		782	802
Pension Benefits [Member] | Fixed Maturity Securities [Member] | Level 1 [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		0	0
Pension Benefits [Member] | Fixed Maturity Securities [Member] | Level 2 [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		773	792
Pension Benefits [Member] | Fixed Maturity Securities [Member] | Level 3 [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		9	10
Pension Benefits [Member] | Corporate Bonds and Other [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		652	597
Pension Benefits [Member] | Corporate Bonds and Other [Member] | Level 2 [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		643	587
Pension Benefits [Member] | Corporate Bonds and Other [Member] | Level 3 [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		9	10
Pension Benefits [Member] | States, Municipalities and Political Subdivisions [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		32	51
Pension Benefits [Member] | States, Municipalities and Political Subdivisions [Member] | Level 2 [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		32	51
Pension Benefits [Member] | Asset-Backed [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		98	154
Pension Benefits [Member] | Asset-Backed [Member] | Level 2 [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		98	154
Pension Benefits [Member] | Equity Securities [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		922	669
Pension Benefits [Member] | Equity Securities [Member] | Level 1 [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		785	541
Pension Benefits [Member] | Equity Securities [Member] | Level 2 [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		137	128
Pension Benefits [Member] | Short-term Investments [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		75	81
Pension Benefits [Member] | Short-term Investments [Member] | Level 1 [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		37	74
Pension Benefits [Member] | Short-term Investments [Member] | Level 2 [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		38	7
Pension Benefits [Member] | Fixed Income Mutual Funds [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		139	128
Pension Benefits [Member] | Fixed Income Mutual Funds [Member] | Level 1 [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		139	128
Pension Benefits [Member] | Other Assets [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		8	20
Pension Benefits [Member] | Other Assets [Member] | Level 1 [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets			11
Pension Benefits [Member] | Other Assets [Member] | Level 2 [Member]
Pension Plan Assets Measured at Fair Value on a Recurring Basis [Abstract]
Fair value of plan assets		$ 8	$ 9

[1] Certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in this table for these investments are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the statement of financial position.