Retirement Benefits - Schedule of Changes in Fair Value of Level 3 Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Change in fair value of assets:
Fair value - beginning of year	$ 932.5
Fair value - end of year	957.0	$ 932.5
Unobservable Inputs (Level 3)
Change in fair value of assets:
Fair value - beginning of year	37.2	27.8
Unrealized Gains (Losses)	9.0	(0.3)
Realized Gains	1.3	0.5
Purchases, Sales, and Settlements, net	(6.4)	9.2
Fair value - end of year	41.1	37.2
Unobservable Inputs (Level 3) | Domestic equity securities
Change in fair value of assets:
Fair value - beginning of year	4.2	4.9
Unrealized Gains (Losses)	1.9	(0.3)
Realized Gains	0.0	0.3
Purchases, Sales, and Settlements, net	0.0	(0.7)
Fair value - end of year	6.1	4.2
Unobservable Inputs (Level 3) | International equity securities
Change in fair value of assets:
Fair value - beginning of year	0.0	0.1
Unrealized Gains (Losses)		(0.1)
Realized Gains		0.0
Purchases, Sales, and Settlements, net		0.0
Fair value - end of year		0.0
Unobservable Inputs (Level 3) | Corporate debt securities
Change in fair value of assets:
Fair value - beginning of year	27.6	21.5
Unrealized Gains (Losses)	5.4	(0.1)
Realized Gains	0.0	0.1
Purchases, Sales, and Settlements, net	(1.1)	6.1
Fair value - end of year	31.9	27.6
Unobservable Inputs (Level 3) | Private assets
Change in fair value of assets:
Fair value - beginning of year	5.4	1.3
Unrealized Gains (Losses)	1.7	0.2
Realized Gains	1.3	0.1
Purchases, Sales, and Settlements, net	(5.3)	3.8
Fair value - end of year	$ 3.1	$ 5.4