Employee Postretirement Benefits (Pension Plan Assets of the Principal Plans Measured at fair value) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Pension Plans
Fair value of plan assets	$ 3,897	$ 3,534	$ 3,508
Principal Plans
Fair value of plan assets	3,312	3,011
Principal Plans \| Fair Value, Inputs, Level 1
Fair value of plan assets	638	240
Principal Plans \| Fair Value, Inputs, Level 3
Fair value of plan assets	334
Principal Plans \| Cash and Cash Equivalents
Fair value of plan assets	28	33
Principal Plans \| Cash and Cash Equivalents \| Mutual and Pooled Funds \| Net Asset Value or Its Equivalent
Fair value of plan assets	24	17
Principal Plans \| Cash and Cash Equivalents \| Fair Value, Inputs, Level 1
Fair value of plan assets	28	33
Principal Plans \| Cash and Cash Equivalents \| Fair Value, Inputs, Level 3
Fair value of plan assets	0
Principal Plans \| U.S. government and municipals
Fair value of plan assets	522	122
Principal Plans \| U.S. government and municipals \| Mutual and Pooled Funds \| Net Asset Value or Its Equivalent
Fair value of plan assets	150	128
Principal Plans \| U.S. government and municipals \| Fair Value, Inputs, Level 1
Fair value of plan assets	522	122
Principal Plans \| U.S. government and municipals \| Fair Value, Inputs, Level 3
Fair value of plan assets	0
Principal Plans \| U.S. corporate debt \| Mutual and Pooled Funds \| Net Asset Value or Its Equivalent
Fair value of plan assets	686	648
Principal Plans \| International Bonds \| Mutual and Pooled Funds \| Net Asset Value or Its Equivalent
Fair value of plan assets	665	1,223
Principal Plans \| U.S. Equity
Fair value of plan assets	41	41
Principal Plans \| U.S. Equity \| Fair Value, Inputs, Level 1
Fair value of plan assets	41	41
Principal Plans \| U.S. Equity \| Fair Value, Inputs, Level 3
Fair value of plan assets	0
Principal Plans \| International Equity
Fair value of plan assets	47	44
Principal Plans \| International Equity \| Fair Value, Inputs, Level 1
Fair value of plan assets	47	44
Principal Plans \| International Equity \| Fair Value, Inputs, Level 3
Fair value of plan assets	0
Principal Plans \| Non-U.S. Equity \| Mutual and Pooled Funds \| Net Asset Value or Its Equivalent
Fair value of plan assets	85	68
Principal Plans \| Global equity \| Mutual and Pooled Funds \| Net Asset Value or Its Equivalent
Fair value of plan assets	730	$ 687
Principal Plans \| Insurance Contract
Fair value of plan assets	334
Principal Plans \| Insurance Contract \| Fair Value, Inputs, Level 1
Fair value of plan assets	0
Principal Plans \| Insurance Contract \| Fair Value, Inputs, Level 3
Fair value of plan assets	$ 334