Employee Postretirement Benefits (Pension Plan Assets of the Principal Plans Measured at fair value) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Principal Plans
Fair value of plan assets	$ 3,303	$ 2,936
Principal Plans | Fair Value, Inputs, Level 1
Fair value of plan assets	258	230
Principal Plans | Fair Value, Inputs, Level 2
Fair value of plan assets	244	225
Principal Plans | Fair Value, Inputs, Level 3
Fair value of plan assets	372	347
Principal Plans | Cash and Cash Equivalents
Fair value of plan assets	44	15
Principal Plans | Cash and Cash Equivalents | Net Asset Value or Its Equivalent
Fair value of plan assets	31	45
Principal Plans | Cash and Cash Equivalents | Fair Value, Inputs, Level 1
Fair value of plan assets	44	15
Principal Plans | U.S. government and municipals
Fair value of plan assets	161	161
Principal Plans | U.S. government and municipals | Net Asset Value or Its Equivalent
Fair value of plan assets	586	479
Principal Plans | U.S. government and municipals | Fair Value, Inputs, Level 1
Fair value of plan assets	147	145
Principal Plans | U.S. government and municipals | Fair Value, Inputs, Level 2
Fair value of plan assets	14	16
Principal Plans | U.S. corporate debt
Fair value of plan assets	221	196
Principal Plans | U.S. corporate debt | Net Asset Value or Its Equivalent
Fair value of plan assets	662	593
Principal Plans | U.S. corporate debt | Fair Value, Inputs, Level 2
Fair value of plan assets	221	196
Principal Plans | International bonds
Fair value of plan assets	9	13
Principal Plans | International bonds | Net Asset Value or Its Equivalent
Fair value of plan assets	504	494
Principal Plans | International bonds | Fair Value, Inputs, Level 2
Fair value of plan assets	9	13
Principal Plans | U.S. Equity
Fair value of plan assets	39	30
Principal Plans | U.S. Equity | Fair Value, Inputs, Level 1
Fair value of plan assets	39	30
Principal Plans | International Equity
Fair value of plan assets	32	31
Principal Plans | International Equity | Fair Value, Inputs, Level 1
Fair value of plan assets	32	31
Principal Plans | Non-U.S. Equity | Net Asset Value or Its Equivalent
Fair value of plan assets	86	73
Principal Plans | Global equity | Net Asset Value or Its Equivalent
Fair value of plan assets	572	448
Principal Plans | Insurance Contract
Fair value of plan assets	372	347
Principal Plans | Insurance Contract | Fair Value, Inputs, Level 3
Fair value of plan assets	372	347
Principal Plans | Other
Fair value of plan assets	(16)	11
Principal Plans | Other | Fair Value, Inputs, Level 1
Fair value of plan assets	(4)	9
Defined Benefit Pension Plans
Fair value of plan assets	$ 3,803	$ 3,398	$ 3,897