Retirement and Other Postretirement Benefit Plans - Schedule of Actual Allocations for the Pension Assets and Target Allocations by Asset Class (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Change in plan assets:
Percentage Of Plan Assets	100.00%	100.00%
Target Allocations	100.00%	100.00%
Diversified Growth Funds
Change in plan assets:
Percentage Of Plan Assets	23.80%	24.10%
Target Allocations	28.10%	29.00%
Index Linked Gilts
Change in plan assets:
Percentage Of Plan Assets	25.80%	21.10%
Target Allocations	12.60%	13.00%
Liability Driven Investments
Change in plan assets:
Percentage Of Plan Assets	19.20%	20.10%
Target Allocations	23.20%	24.00%
All Other Regions Equity Fund
Change in plan assets:
Percentage Of Plan Assets	16.60%	18.70%
Target Allocations	19.60%	34.00%
U.K. Equity Fund
Change in plan assets:
Percentage Of Plan Assets	11.20%	12.60%
Target Allocations	13.30%
Diversified Investment, Funds
Change in plan assets:
Percentage Of Plan Assets	1.20%	1.20%
Target Allocations	1.20%
Other Insurance Product Line
Change in plan assets:
Percentage Of Plan Assets	2.10%	2.10%
Target Allocations	2.00%
Cash and Cash Equivalents
Change in plan assets:
Percentage Of Plan Assets	0.10%	0.10%