Retirement Benefits - Changes in Fair Value of Level 3 Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at the beginning of the period	$ 480.2
Actual return on assets:
Plan assets at end of period	446.9	$ 480.2
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at the beginning of the period	29.9
Actual return on assets:
Plan assets at end of period	32.6	29.9
Insurance Contracts
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at the beginning of the period	29.9
Actual return on assets:
Plan assets at end of period	32.6	29.9
Insurance Contracts | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at the beginning of the period	29.9	30.2
Actual return on assets:
Related to assets held at reporting date	2.7	(0.3)
Related to assets sold during the period	0.0	0.0
Purchases, sales, issuances and settlements	0.0	0.0
Plan assets at end of period	$ 32.6	$ 29.9