Pension and Other Postretirement Benefits - Assumptions Used (Details) - United States [Member] - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Assumptions for pension and other postretirement benefits [Abstract]
Mortality table for pension and postretirement benefit obligations, current year	RP 2014 with MP 2017 projection scale
Mortality table for pension and postretirement benefit obligations, prior year	RP 2014 with MP 2016 projection scale
Defined Benefit Plan Assumed Health Care Cost Trend Rates [Abstract]
Estimated rate of increase in postretirement health care costs, next fiscal year	6.50%
Annual decrease in estimated rate of increase in postretirement health care costs	0.25%
Estimated rate of increase in postretirement health care costs, ultimate	4.75%
Year in which ultimate estimated rate of increase in postretirement health care costs is reached	2024
Pension Benefits [Member]
Assumptions for pension and other postretirement benefits [Abstract]
Discount rate: Benefit obligation	3.51%	3.95%
Discount rate: Benefit cost	3.95%	4.17%
Expected long-term return on plan assets: Benefit cost	7.25%	7.25%
Other Postretirement Benefits [Member]
Assumptions for pension and other postretirement benefits [Abstract]
Discount rate: Benefit obligation	3.37%	3.70%
Discount rate: Benefit cost	3.70%	3.89%
Expected long-term return on plan assets: Benefit cost	5.00%	5.00%
Impact of a 1% increase or decrease in estimate rate of future increases in postretirement health care benefits [Abstract]
Effect of increase in estimated rate on postretirement cost
Effect of decrease in estimated rate on postretirement cost
Effect of increase in estimated rate on postretirement benefit obligation
Effect of decrease in estimated rate on postretirement benefit obligation