Retirement Benefits - Projected Benefit Obligation Assumptions (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.67%	4.12%	4.71%
Rate of compensation increase	3.24%	3.55%	3.59%
Discount rate	4.12%	4.71%	4.23%
Expected return on plan assets	4.94%	6.04%	6.36%
Rate of compensation increase	3.55%	3.59%	3.15%
Other Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.70%	4.10%	4.60%
Rate of compensation increase	4.00%	3.93%	3.92%
Discount rate	4.10%	4.60%	4.10%
Rate of compensation increase	3.93%	3.92%	3.60%