Employee Postretirement Benefits (Pension Plan Assets of the Principal Plans Measured at fair value) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Pension Plans
Fair value of plan assets	$ 3,534	$ 3,508	$ 5,914
Principal Plans
Fair value of plan assets	3,011	3,019
Principal Plans | Fair Value, Inputs, Level 1
Fair value of plan assets	240	241
Principal Plans | Fair Value, Inputs, Level 2
Fair value of plan assets	0	39
Principal Plans | Cash and Cash Equivalents
Fair value of plan assets	33	14
Principal Plans | Cash and Cash Equivalents | Mutual and Pooled Funds | Net Asset Value or Its Equivalent
Fair value of plan assets	17	34
Principal Plans | Cash and Cash Equivalents | Fair Value, Inputs, Level 1
Fair value of plan assets	33	12
Principal Plans | Cash and Cash Equivalents | Fair Value, Inputs, Level 1 | Mutual and Pooled Funds
Fair value of plan assets	0	0
Principal Plans | Cash and Cash Equivalents | Fair Value, Inputs, Level 2
Fair value of plan assets	0	2
Principal Plans | Cash and Cash Equivalents | Fair Value, Inputs, Level 2 | Mutual and Pooled Funds
Fair value of plan assets	0	0
Principal Plans | U.S. government and municipals
Fair value of plan assets	122	157
Principal Plans | U.S. government and municipals | Mutual and Pooled Funds | Net Asset Value or Its Equivalent
Fair value of plan assets	128	149
Principal Plans | U.S. government and municipals | Fair Value, Inputs, Level 1
Fair value of plan assets	122	141
Principal Plans | U.S. government and municipals | Fair Value, Inputs, Level 1 | Mutual and Pooled Funds
Fair value of plan assets	0	0
Principal Plans | U.S. government and municipals | Fair Value, Inputs, Level 2
Fair value of plan assets	0	16
Principal Plans | U.S. government and municipals | Fair Value, Inputs, Level 2 | Mutual and Pooled Funds
Fair value of plan assets	0	0
Principal Plans | U.S. corporate debt
Fair value of plan assets		21
Principal Plans | U.S. corporate debt | Mutual and Pooled Funds | Net Asset Value or Its Equivalent
Fair value of plan assets	648	623
Principal Plans | U.S. corporate debt | Fair Value, Inputs, Level 1
Fair value of plan assets		0
Principal Plans | U.S. corporate debt | Fair Value, Inputs, Level 1 | Mutual and Pooled Funds
Fair value of plan assets	0	0
Principal Plans | U.S. corporate debt | Fair Value, Inputs, Level 2
Fair value of plan assets		21
Principal Plans | U.S. corporate debt | Fair Value, Inputs, Level 2 | Mutual and Pooled Funds
Fair value of plan assets	0	0
Principal Plans | International Bonds | Mutual and Pooled Funds | Net Asset Value or Its Equivalent
Fair value of plan assets	1,223	1,236
Principal Plans | International Bonds | Fair Value, Inputs, Level 1 | Mutual and Pooled Funds
Fair value of plan assets	0	0
Principal Plans | International Bonds | Fair Value, Inputs, Level 2 | Mutual and Pooled Funds
Fair value of plan assets	0	0
Principal Plans | U.S. Equity
Fair value of plan assets	41	58
Principal Plans | U.S. Equity | Fair Value, Inputs, Level 1
Fair value of plan assets	41	58
Principal Plans | U.S. Equity | Fair Value, Inputs, Level 2
Fair value of plan assets	0	0
Principal Plans | International Equity
Fair value of plan assets	44	30
Principal Plans | International Equity | Fair Value, Inputs, Level 1
Fair value of plan assets	44	30
Principal Plans | International Equity | Fair Value, Inputs, Level 2
Fair value of plan assets	0	0
Principal Plans | Non-U.S. Equity | Mutual and Pooled Funds | Net Asset Value or Its Equivalent
Fair value of plan assets	68	67
Principal Plans | Non-U.S. Equity | Fair Value, Inputs, Level 1 | Mutual and Pooled Funds
Fair value of plan assets	0	0
Principal Plans | Non-U.S. Equity | Fair Value, Inputs, Level 2 | Mutual and Pooled Funds
Fair value of plan assets	0	0
Principal Plans | Global equity | Mutual and Pooled Funds | Net Asset Value or Its Equivalent
Fair value of plan assets	687	630
Principal Plans | Global equity | Fair Value, Inputs, Level 1 | Mutual and Pooled Funds
Fair value of plan assets	0	0
Principal Plans | Global equity | Fair Value, Inputs, Level 2 | Mutual and Pooled Funds
Fair value of plan assets	$ 0	$ 0