Employee Pension and Other Postretirement Benefits - Periodic Benefit Costs, Forecasted (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Net actuarial loss		$ 109
Prior service costs (credits)		(18)
Amounts to be reclassified from accumulated other comprehensive loss		$ 91
defined benefit plan, assumptions used calculating net periodic benefit cost, interest cost	3.72%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	7.50%	7.50%	7.50%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase		4.70%	4.70%
Defined Benefit Plan, Net Periodic Benefit Cost		$ 79	$ 40	$ 127
Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Net actuarial loss		5
Prior service costs (credits)		(8)
Amounts to be reclassified from accumulated other comprehensive loss		(3)
Forecast | Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Net actuarial loss		76
Prior service costs (credits)		(18)
Amounts to be reclassified from accumulated other comprehensive loss		58
DefinedBenefitPlanEffectOfFifityBasisPointIncreaseOnDiscountRate	$ 19
DefinedBenefitPlanEffectOfFifityBasisPointIncreaseOnExpectedReturnofPlanAssets	16
DefinedBenefitPlanEffectOfFifityBasisPointIncreaseOnFutureCompensationLevels	2
DefinedBenefitPlanEffectOfFifityBasisPointDecreaseOnDiscountRate	21
DefinedBenefitPlanEffectOfFifityBasisPointDecreaseOnExpectedReturnofPlanAssets	16
DefinedBenefitPlanEffectOfFifityBasisPointDecreaseOnFutureCompensationLevels	$ 2
Forecast | Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Net actuarial loss		10
Prior service costs (credits)		(8)
Amounts to be reclassified from accumulated other comprehensive loss		$ 2
Subsequent Event [Member] | Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	4.15%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase	4.70%
Defined Benefit Plan, Net Periodic Benefit Cost	$ 17