Employee Pension and Other Postretirement Benefits - Fair Value Hierarchy of Assets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	$ 2,099	$ 2,136
Investments measured at net asset value	1,420	1,704
Investments at fair value	3,519	3,840
Accrued dividends and interest receivable	12	8
Net pending trades	(2)	6
Total plan assets	3,529	3,854	$ 3,254
Pension Benefits | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	805	1,216
Pension Benefits | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	1,294	920
Pension Benefits | Level 3
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Pension Benefits | Cash and cash equivalents
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	25	26
Pension Benefits | Cash and cash equivalents | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Pension Benefits | Cash and cash equivalents | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	25	26
Pension Benefits | Cash and cash equivalents | Level 3
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Pension Benefits | U.S. Government securities
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	382	195
Pension Benefits | U.S. Government securities | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Pension Benefits | U.S. Government securities | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	382	195
Pension Benefits | U.S. Government securities | Level 3
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Pension Benefits | Corporate bonds
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	824	638
Pension Benefits | Corporate bonds | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Pension Benefits | Corporate bonds | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	824	638
Pension Benefits | Corporate bonds | Level 3
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Pension Benefits | Equity securities
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	191	388
Pension Benefits | Equity securities | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	185	381
Pension Benefits | Equity securities | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	6	7
Pension Benefits | Equity securities | Level 3
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Pension Benefits | Mutual funds
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	620	830
Pension Benefits | Mutual funds | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	620	830
Pension Benefits | Mutual funds | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Pension Benefits | Mutual funds | Level 3
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Pension Benefits | Municipal bonds
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	37	43
Pension Benefits | Municipal bonds | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Pension Benefits | Municipal bonds | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	37	43
Pension Benefits | Municipal bonds | Level 3
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Pension Benefits | Other
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	20	16
Pension Benefits | Other | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	5
Pension Benefits | Other | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	20	11
Pension Benefits | Other | Level 3
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	124	143
Investments measured at net asset value	134	157
Investments at fair value	258	300
Net pending trades	(3)	(12)
Total plan assets	255	288	260
Other Benefits | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	54	66
Other Benefits | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	70	77
Other Benefits | Level 3
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Other Benefits | Cash and cash equivalents
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy			3
Investments at fair value	1
Other Benefits | Cash and cash equivalents | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy		0
Other Benefits | Cash and cash equivalents | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy		3
Investments at fair value	1
Other Benefits | Cash and cash equivalents | Level 3
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy		0
Other Benefits | U.S. Government securities
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	37	44
Other Benefits | U.S. Government securities | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Other Benefits | U.S. Government securities | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	37	44
Other Benefits | U.S. Government securities | Level 3
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Other Benefits | Corporate bonds
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	32	29
Other Benefits | Corporate bonds | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Other Benefits | Corporate bonds | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	32	29
Other Benefits | Corporate bonds | Level 3
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Other Benefits | Equity securities
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0		$ 1
Other Benefits | Equity securities | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Other Benefits | Equity securities | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy		1
Investments at fair value	0
Other Benefits | Equity securities | Level 3
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Other Benefits | Mutual funds
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	54	66
Other Benefits | Mutual funds | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	54	66
Other Benefits | Mutual funds | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Other Benefits | Mutual funds | Level 3
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	$ 0	$ 0