Retirement Benefits - Changes in Fair Value of Level 3 Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at the beginning of the period	$ 507.4
Actual return on assets:
Plan assets at end of period	480.2	$ 507.4
Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at the beginning of the period	30.2
Actual return on assets:
Plan assets at end of period	29.9	30.2
Insurance Contracts
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at the beginning of the period	30.2
Actual return on assets:
Plan assets at end of period	29.9	30.2
Insurance Contracts | Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at the beginning of the period	30.2	23.9
Actual return on assets:
Related to assets held at reporting date	(0.3)	3.9
Related to assets sold during the period	0.0	0.0
Related to assets acquired by acquisition		2.4
Purchases, sales, issuances and settlements	0.0	0.0
Plan assets at end of period	$ 29.9	$ 30.2