Retirement Benefits - Schedule of Changes in Fair Value of Level 3 Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Change in fair value of assets:
Fair value - beginning of period	$ 925.1
Fair value - end of period	931.2	$ 925.1
Significant Unobservable Inputs (Level 3)
Change in fair value of assets:
Fair value - beginning of period	9.5	1.2
Unrealized Gains (Losses) on Plan Assets	6.1	0.1
Purchases, Sales, and Settlements, net	25.2	8.2
Fair value - end of period	40.8	9.5
Significant Unobservable Inputs (Level 3) | Real estate investments
Change in fair value of assets:
Fair value - beginning of period	0.5	0.7
Unrealized Gains (Losses) on Plan Assets	0.0	0.0
Purchases, Sales, and Settlements, net	(0.5)	(0.2)
Fair value - end of period	0.0	0.5
Significant Unobservable Inputs (Level 3) | Private assets
Change in fair value of assets:
Fair value - beginning of period	0.0
Unrealized Gains (Losses) on Plan Assets	1.2
Purchases, Sales, and Settlements, net	24.5
Fair value - end of period	25.7	0.0
Significant Unobservable Inputs (Level 3) | Cash and cash equivalents | Domestic equity securities
Change in fair value of assets:
Fair value - beginning of period	2.2	0.5
Unrealized Gains (Losses) on Plan Assets	2.9	0.1
Purchases, Sales, and Settlements, net	(1.3)	1.6
Fair value - end of period	3.8	2.2
Significant Unobservable Inputs (Level 3) | Fixed income | Corporate debt securities
Change in fair value of assets:
Fair value - beginning of period	6.8	0.0
Unrealized Gains (Losses) on Plan Assets	2.0	0.0
Purchases, Sales, and Settlements, net	2.5	6.8
Fair value - end of period	$ 11.3	$ 6.8