Employee Benefits - Fair Values of Pension Plan Assets by Asset Category (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Foreign Government Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	$ 2	$ 1
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	1,163	1,133	$ 869
Pension Plan [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	14
Pension Plan [Member] | Equity Securities, Common, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	11	133
Pension Plan [Member] | Equity Securities, Common, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	1	15
Pension Plan [Member] | Equity Securities, Preferred, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	3	2
Pension Plan [Member] | Corporate Bonds - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	152	90
Pension Plan [Member] | Corporate Bonds - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	61	26
Pension Plan [Member] | Government Bonds and Agency Bonds - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	203	101
Pension Plan [Member] | State and Municipal Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	31	5
Pension Plan [Member] | Mutual Funds, Equity, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	118
Pension Plan [Member] | Mutual Funds, Equity, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	7	35
Pension Plan [Member] | Mutual Funds, Fixed Income, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	152
Pension Plan [Member] | Mutual Funds, Fixed Income, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	19
Pension Plan [Member] | Mutual Funds, Real Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	9
Pension Plan [Member] | Collective Investment Funds, Equity, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	45	15
Pension Plan [Member] | Collective Investment Funds, Equity, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	41	97
Pension Plan [Member] | Convertible Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	43
Pension Plan [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	456	79
Pension Plan [Member] | Short-term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	16	18
Pension Plan [Member] | Real Assets, Collective Investment Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	70	95
Pension Plan [Member] | Insurance Investment Contracts and Pooled Separate Accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets measured at NAV	14	14
Pension Plan [Member] | Other Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets measured at NAV	50	52
Level 1 [Member] | Foreign Government Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 1 [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	22	497
Level 1 [Member] | Pension Plan [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	14
Level 1 [Member] | Pension Plan [Member] | Equity Securities, Common, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	11	133
Level 1 [Member] | Pension Plan [Member] | Equity Securities, Common, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	1	15
Level 1 [Member] | Pension Plan [Member] | Equity Securities, Preferred, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	3	2
Level 1 [Member] | Pension Plan [Member] | Corporate Bonds - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 1 [Member] | Pension Plan [Member] | Corporate Bonds - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 1 [Member] | Pension Plan [Member] | Government Bonds and Agency Bonds - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 1 [Member] | Pension Plan [Member] | State and Municipal Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 1 [Member] | Pension Plan [Member] | Mutual Funds, Equity, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	118
Level 1 [Member] | Pension Plan [Member] | Mutual Funds, Equity, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	7	35
Level 1 [Member] | Pension Plan [Member] | Mutual Funds, Fixed Income, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	152
Level 1 [Member] | Pension Plan [Member] | Mutual Funds, Fixed Income, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	19
Level 1 [Member] | Pension Plan [Member] | Mutual Funds, Real Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	9
Level 1 [Member] | Pension Plan [Member] | Collective Investment Funds, Equity, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 1 [Member] | Pension Plan [Member] | Collective Investment Funds, Equity, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 1 [Member] | Pension Plan [Member] | Convertible Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 1 [Member] | Pension Plan [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 1 [Member] | Pension Plan [Member] | Short-term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 1 [Member] | Pension Plan [Member] | Real Assets, Collective Investment Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 2 [Member] | Foreign Government Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	2	1
Level 2 [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	1,077	570
Level 2 [Member] | Pension Plan [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 2 [Member] | Pension Plan [Member] | Equity Securities, Common, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 2 [Member] | Pension Plan [Member] | Equity Securities, Common, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 2 [Member] | Pension Plan [Member] | Equity Securities, Preferred, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 2 [Member] | Pension Plan [Member] | Corporate Bonds - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	152	90
Level 2 [Member] | Pension Plan [Member] | Corporate Bonds - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	61	26
Level 2 [Member] | Pension Plan [Member] | Government Bonds and Agency Bonds - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	203	101
Level 2 [Member] | Pension Plan [Member] | State and Municipal Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	31	5
Level 2 [Member] | Pension Plan [Member] | Mutual Funds, Equity, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 2 [Member] | Pension Plan [Member] | Mutual Funds, Equity, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 2 [Member] | Pension Plan [Member] | Mutual Funds, Fixed Income, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 2 [Member] | Pension Plan [Member] | Mutual Funds, Fixed Income, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 2 [Member] | Pension Plan [Member] | Mutual Funds, Real Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 2 [Member] | Pension Plan [Member] | Collective Investment Funds, Equity, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	45	15
Level 2 [Member] | Pension Plan [Member] | Collective Investment Funds, Equity, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	41	97
Level 2 [Member] | Pension Plan [Member] | Convertible Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	43
Level 2 [Member] | Pension Plan [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	456	79
Level 2 [Member] | Pension Plan [Member] | Short-term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	16	18
Level 2 [Member] | Pension Plan [Member] | Real Assets, Collective Investment Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	70	95
Level 3 [Member] | Foreign Government Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Pension Plan [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Pension Plan [Member] | Equity Securities, Common, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Pension Plan [Member] | Equity Securities, Common, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Pension Plan [Member] | Equity Securities, Preferred, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Pension Plan [Member] | Corporate Bonds - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Pension Plan [Member] | Corporate Bonds - International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Pension Plan [Member] | Government Bonds and Agency Bonds - U.S. [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Pension Plan [Member] | State and Municipal Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Pension Plan [Member] | Mutual Funds, Equity, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Pension Plan [Member] | Mutual Funds, Equity, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Pension Plan [Member] | Mutual Funds, Fixed Income, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Pension Plan [Member] | Mutual Funds, Fixed Income, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Pension Plan [Member] | Mutual Funds, Real Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Pension Plan [Member] | Collective Investment Funds, Equity, United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Pension Plan [Member] | Collective Investment Funds, Equity, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Pension Plan [Member] | Convertible Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Pension Plan [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Pension Plan [Member] | Short-term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	0	0
Level 3 [Member] | Pension Plan [Member] | Real Assets, Collective Investment Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	$ 0	$ 0