Employee Pension and Other Postretirement Benefits - Fair Value Hierarchy of Assets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	$ 3,450	$ 2,099
Investments measured at net asset value	680	1,420
Investments at fair value	4,130	3,519
Accrued dividends and interest receivable	10	12
Net pending trades	0	(2)
Total plan assets	4,140	3,529	$ 3,854
Pension Benefits | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	997	805
Pension Benefits | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	2,453	1,294
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	11	25
Pension Benefits | Cash and cash equivalents | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	9	0
Pension Benefits | Cash and cash equivalents | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	2	25
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 | Defined Benefit Plan, Common Collective Trust [Member]
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	1,089
Pension Benefits | Defined Benefit Plan, Common Collective Trust [Member] | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0
Pension Benefits | Defined Benefit Plan, Common Collective Trust [Member] | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	1,089
Pension Benefits | Defined Benefit Plan, Common Collective Trust [Member] | Level 3
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0
Pension Benefits | U.S. Government securities
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	419	382
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	419	382
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	882	824
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	882	824
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	252	191
Pension Benefits | Equity securities | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	246	185
Pension Benefits | Equity securities | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	6	6
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	746	620
Pension Benefits | Mutual funds | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	746	620
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	38	37
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	38	37
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	13	20
Pension Benefits | Other | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	(4)	0
Pension Benefits | Other | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	17	20
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	293	124
Investments measured at net asset value	0	134
Investments at fair value	293	258
Net pending trades	(2)	(3)
Total plan assets	291	255	$ 288
Other Benefits | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	65	54
Other Benefits | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	228	70
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	0	1
Other Benefits | Cash and cash equivalents | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Other Benefits | Cash and cash equivalents | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	1
Other Benefits | Cash and cash equivalents | Level 3
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	0
Other Benefits | Defined Benefit Plan, Common Collective Trust [Member]
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	103
Other Benefits | Defined Benefit Plan, Common Collective Trust [Member] | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0
Other Benefits | Defined Benefit Plan, Common Collective Trust [Member] | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	103
Other Benefits | Defined Benefit Plan, Common Collective Trust [Member] | 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	45	37
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	45	37
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	27	32
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	27	32
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	0
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		0
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	65	54
Other Benefits | Mutual funds | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	65	54
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
Other Benefits | Pooled Separate Accounts [Member]
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	53
Other Benefits | Pooled Separate Accounts [Member] | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0
Other Benefits | Pooled Separate Accounts [Member] | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	53
Other Benefits | Pooled Separate Accounts [Member] | Level 3
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	$ 0