Retirement Benefits - Schedule of Changes in Fair Value of Level 3 Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Change in fair value of assets:
Fair value - beginning of year	$ 957.0
Fair value - end of year	1,005.0	$ 957.0
Unobservable Inputs (Level 3)
Change in fair value of assets:
Fair value - beginning of year	41.1	37.2
Unrealized Gains	10.2	9.0
Realized Gains	1.1	1.3
Purchases, Sales, and Settlements, net	(33.6)	(6.4)
Fair value - end of year	18.8	41.1
Unobservable Inputs (Level 3) | Equity securities
Change in fair value of assets:
Fair value - beginning of year	6.1	4.2
Unrealized Gains	10.1	1.9
Realized Gains	1.1	0.0
Purchases, Sales, and Settlements, net	(17.3)	0.0
Fair value - end of year	0.0	6.1
Unobservable Inputs (Level 3) | Corporate debt securities
Change in fair value of assets:
Fair value - beginning of year	31.9	27.6
Unrealized Gains	0.1	5.4
Realized Gains	0.0	0.0
Purchases, Sales, and Settlements, net	(14.4)	(1.1)
Fair value - end of year	17.6	31.9
Unobservable Inputs (Level 3) | Private assets
Change in fair value of assets:
Fair value - beginning of year	3.1	5.4
Unrealized Gains	0.0	1.7
Realized Gains	0.0	1.3
Purchases, Sales, and Settlements, net	(1.9)	(5.3)
Fair value - end of year	$ 1.2	$ 3.1