Retirement Benefits - Schedule of One Percentage Point Change in Assumptions Effects on Projected Benefit Obligations (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Retirement Benefits [Abstract]
Effect of 1% decrease on net periodic benefit expense components	$ 21.7
Effect of 1% increase on net periodic benefit expense components	(18.5)
Effect of 1% decrease on projected benefit obligation	153.5
Effect of 1% increase on projected benefit obligation	(128.9)
Effect of 1% decrease on net periodic pension benefit expense components	9.2
Effect of 1% increase on net periodic pension benefit expense components	(9.2)
Effect of 1% decrease on net periodic medical and life insurance benefit expense	(0.2)
Effect of 1% increase on net periodic medical and life insurance benefit expense	0.3
Effect of 1% decrease on accumulated benefit obligation	(0.7)
Effect of 1% increase on accumulated benefit obligation	$ 0.8