Retirement Benefits - Narrative (Details)	12 Months Ended
	Mar. 31, 2019 USD ($) category Rate	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarial gains	$ 400,000	$ 3,300,000	$ 2,600,000
Number of plans | plan				3
Contributions	1,300,000	2,900,000	4,900,000
Plan assets	480,200,000	507,400,000
Multi-employer and government-sponsored plans - Expense recognized			200,000
Defined contribution savings plan - expense recognized	15,500,000	15,000,000	12,400,000
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarial gains	(1,700,000)	(7,100,000)
Contributions	4,000,000	5,500,000
Projected benefit obligation	623,800,000	652,500,000	659,300,000
Plan assets	480,200,000	507,400,000	513,000,000
Under-funded status	143,600,000	145,100,000
Future amortization of prior service cost (credit)	$ 0
Number of investment categories | category	2
Pension plans that are not fully funded - projected benefit obligation	$ 591,000,000	618,600,000
Pension plans that are not fully funded - accumulated benefit obligation	586,500,000	613,600,000
Pension plans that are not fully funded - fair value of plan assets	446,800,000	472,800,000
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarial gains	(2,500,000)	(2,800,000)
Contributions	2,500,000	3,000,000
Projected benefit obligation	16,900,000	21,500,000	25,700,000
Plan assets	0	0	$ 0
Under-funded status	16,900,000	$ 21,500,000
Future amortization of prior service cost (credit)	$ (300,000)
Assumed health care cost trend rate, percent | Rate	6.30%
Ultimate health care cost trend rate, percent | Rate	5.00%