Employee Pension and Other Postretirement Benefits - Fair Value Hierarchy of Assets (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	$ 1,763	$ 2,495
Investments measured at net asset value	1,493	510
Investments at fair value	3,256	3,005
Accrued dividends and interest receivable	7	6
Net pending trades	(9)	(19)
Total plan assets	3,254	2,992	$ 2,897
Pension Benefits | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	1,020	1,726
Pension Benefits | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	743	769
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	9	19
Pension Benefits | Cash and cash equivalents | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	0	3
Pension Benefits | Cash and cash equivalents | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	9	16
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	219	248
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	219	248
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	476	476
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	476	476
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	339	293
Pension Benefits | Equity securities | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	333	284
Pension Benefits | Equity securities | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	6	9
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	685	1,435
Pension Benefits | Mutual funds | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	685	1,435
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	22	7
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	22	7
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	17
Pension Benefits | Other | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	2	4
Pension Benefits | Other | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	11	13
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	194	197
Investments measured at net asset value	72	62
Investments at fair value	266	259
Net pending trades	(6)	(5)
Total plan assets	260	254	$ 257
Other Benefits | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	128	134
Other Benefits | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	66	63
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
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
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	40	34
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	40	34
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	26	24
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	26	24
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		1
Other Benefits | Equity securities | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy		0
Other Benefits | Equity securities | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy		1
Other Benefits | Equity securities | Level 3
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy		0
Other Benefits | Mutual funds
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	128	134
Other Benefits | Mutual funds | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy	128	134
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 | Municipal bonds
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy		1
Other Benefits | Municipal bonds | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy		0
Other Benefits | Municipal bonds | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy		1
Other Benefits | Municipal bonds | Level 3
Pension Plan and Other Postretirement Benefits Plan
Total investments in the fair value hierarchy		$ 0