Retirement Benefits - Narrative - Plan Assumptions (Details) - Medical and Life Insurance Benefits - Participant	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 30, 2015
Defined Benefit Plan Disclosure [Line Items]
Initial healthcare trend rate (pre 65/post 65)	6.50%	7.00%
Number of participants	65
Number of years until rate reaches ultimate trend rate	3 years
Ultimate healthcare trend rate	5.00%	5.00%	5.00%	5.00%