Retirement Plans and Postretirement Medical Benefits (Target Asset Allocation) (Details)	Dec. 31, 2017	Dec. 31, 2016
United States
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	100.00%
Percent of Plan Assets	100.00%	100.00%
United States | U.S. equities
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	15.00%
Percent of Plan Assets	15.00%	17.00%
United States | Non-U.S. equities
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	15.00%
Percent of Plan Assets	15.00%	13.00%
United States | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	60.00%
Percent of Plan Assets	62.00%	60.00%
United States | Real estate
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	5.00%
Percent of Plan Assets	6.00%	6.00%
United States | Private equity
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	5.00%
Percent of Plan Assets	2.00%	4.00%
Foreign
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	100.00%
Percent of Plan Assets	100.00%	100.00%
Foreign | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	40.00%
Percent of Plan Assets	41.00%	41.00%
Foreign | Real estate
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	10.00%
Percent of Plan Assets	9.00%	8.00%
Foreign | U.K. equities
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	10.00%
Percent of Plan Assets	10.00%	22.00%
Foreign | Non-U.K. equities
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	30.00%
Percent of Plan Assets	29.00%	19.00%
Foreign | Diversified growth
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	10.00%
Percent of Plan Assets	9.00%	9.00%
Foreign | Cash
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	0.00%
Percent of Plan Assets	2.00%	1.00%