Retirement Benefits Plans - Other Postretirement Benefits Plan Assumptions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates [Abstract]
Effect on total service and interest cost, 1% increase	$ 1
Effect on total service and interest cost, 1% decrease	(1)
Effect on other postretirement liabilities, 1% increase	17
Effect on other postretirement liabilities, 1% decrease	$ (15)
Other postretirement benefits plans [Member]
Assumptions used to determine benefit obligation at year-end
Discount rate	3.96%	4.04%	3.79%
Health care cost trend rate assumed for next year	7.35%	7.10%	6.31%
Ultimate health care cost trend rate	4.75%	4.75%	4.77%
Year ultimate health care cost trend rate is achieved	2026	2025	2024
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate To Determine Benefit Obligation	4.04%	3.79%	4.48%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate To Determine Service Costs	4.26%	3.79%	4.48%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate To Determine Interest Costs	3.12%	3.79%	4.48%
Assumptions used to determine expense
Initial health care cost trend rate	7.10%	6.31%	6.64%
Ultimate health care cost trend rate	4.75%	4.77%	4.77%
Year ultimate health care cost trend rate is achieved	2025	2024	2023