Employee Pension and Other Postretirement Benefits - Fair Value Hierarchy of Assets (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	$ 3,005	$ 2,934
Accrued dividends and interest receivable	6	7
Net pending trades	(19)	(44)
Total Plan Assets	2,992	2,897	$ 2,716
Pension Benefits | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	1,726	1,724
Pension Benefits | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	941	912
Pension Benefits | Level 3
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	338	298	264
Pension Benefits | Cash and cash equivalents
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	39	53
Pension Benefits | Cash and cash equivalents | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	3	1
Pension Benefits | Cash and cash equivalents | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	36	52
Pension Benefits | U.S. Government securities
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	248	251
Pension Benefits | U.S. Government securities | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	248	251
Pension Benefits | Corporate bonds
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	476	442
Pension Benefits | Corporate bonds | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	476	442
Pension Benefits | Equity securities
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	293	301
Pension Benefits | Equity securities | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	284	295
Pension Benefits | Equity securities | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	9
Pension Benefits | Real Estate Funds [Member]
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	310	270
Pension Benefits | Real Estate Funds [Member] | Level 3
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	310	270
Pension Benefits | Limited partnership
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	180	166
Pension Benefits | Limited partnership | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	152	138
Pension Benefits | Limited partnership | Level 3
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	28	28	22
Pension Benefits | Mutual funds
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	1,435	1,422
Pension Benefits | Mutual funds | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	1,435	1,422
Pension Benefits | Other
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	24	29
Pension Benefits | Other | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	4	6
Pension Benefits | Other | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	20	23
Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	259	264
Net pending trades	(5)	(7)
Total Plan Assets	254	257	$ 245
Other Benefits | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	134	136
Other Benefits | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	125	128
Other Benefits | Cash and cash equivalents
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	3	3
Other Benefits | Cash and cash equivalents | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	3	3
Other Benefits | U.S. Government securities
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	34	36
Other Benefits | U.S. Government securities | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	34	36
Other Benefits | Corporate bonds
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	24	23
Other Benefits | Corporate bonds | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	24	23
Other Benefits | Municipal Bonds
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	1	1
Other Benefits | Municipal Bonds | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	1	1
Other Benefits | Equity securities
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	1
Other Benefits | Equity securities | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	1
Other Benefits | Mutual funds
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	134	136
Other Benefits | Mutual funds | Level 1
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	134	136
Other Benefits | Pooled separate account
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	62	65
Other Benefits | Pooled separate account | Level 2
Pension Plan and Other Postretirement Benefits Plan
Total Plan Investments	$ 62	$ 65