Retirement Benefits - Narrative - Plan Results (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Retirement Benefits [Abstract]
Defined benefit plan, accumulated benefit obligation	$ 1,442.9	$ 1,492.1
Difference between expected return and actual return on market-related value of assets which smoothes asset values (in years)	3 years