Employee Postretirement Benefits (Pension Plan Assets of the Principal Plans Measured at fair value) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Principal Plans
Fair value of plan assets	$ 3,627	$ 3,303
Principal Plans | Fair Value, Inputs, Level 1
Fair value of plan assets	277	258
Principal Plans | Fair Value, Inputs, Level 2
Fair value of plan assets	372	244
Principal Plans | Fair Value, Inputs, Level 3
Fair value of plan assets	380	372
Principal Plans | Cash and Cash Equivalents
Fair value of plan assets	56	44
Principal Plans | Cash and Cash Equivalents | Net Asset Value or Its Equivalent
Fair value of plan assets	15	31
Principal Plans | Cash and Cash Equivalents | Fair Value, Inputs, Level 1
Fair value of plan assets	56	44
Principal Plans | U.S. government and municipals
Fair value of plan assets	191	161
Principal Plans | U.S. government and municipals | Net Asset Value or Its Equivalent
Fair value of plan assets	167	153
Principal Plans | U.S. government and municipals | Fair Value, Inputs, Level 1
Fair value of plan assets	171	147
Principal Plans | U.S. government and municipals | Fair Value, Inputs, Level 2
Fair value of plan assets	20	14
Principal Plans | U.S. corporate debt
Fair value of plan assets	310	221
Principal Plans | U.S. corporate debt | Net Asset Value or Its Equivalent
Fair value of plan assets	730	662
Principal Plans | U.S. corporate debt | Fair Value, Inputs, Level 2
Fair value of plan assets	310	221
Principal Plans | International bonds
Fair value of plan assets	42	9
Principal Plans | International bonds | Net Asset Value or Its Equivalent
Fair value of plan assets	1,062	937
Principal Plans | International bonds | Fair Value, Inputs, Level 2
Fair value of plan assets	42	9
Principal Plans | U.S. Equity
Fair value of plan assets	18	39
Principal Plans | U.S. Equity | Fair Value, Inputs, Level 1
Fair value of plan assets	18	39
Principal Plans | International Equity
Fair value of plan assets	31	32
Principal Plans | International Equity | Fair Value, Inputs, Level 1
Fair value of plan assets	31	32
Principal Plans | Non-U.S. Equity | Net Asset Value or Its Equivalent
Fair value of plan assets	107	86
Principal Plans | Global equity | Net Asset Value or Its Equivalent
Fair value of plan assets	517	572
Principal Plans | Insurance Contract
Fair value of plan assets	380	372
Principal Plans | Insurance Contract | Fair Value, Inputs, Level 3
Fair value of plan assets	380	372
Principal Plans | Other
Fair value of plan assets	1	(16)
Principal Plans | Other | Fair Value, Inputs, Level 1
Fair value of plan assets	1	(4)
Defined Benefit Pension Plans
Fair value of plan assets	$ 4,193	$ 3,803	$ 3,398