Net Income Per Common Share (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 349.4	$ 570.6	$ 355.9
Basic weighted average common shares outstanding (in shares)	259,600,000	238,900,000	236,200,000
Dilutive effect of stock options and RSUs (in shares)	3,500,000	3,400,000	3,800,000
Diluted weighted average common shares outstanding (in shares)	270,600,000	256,200,000	249,900,000
Basic net income per common share (in dollars per share)	$ 1.35	$ 2.39	$ 1.51
Diluted net income per common share (in dollars per share)	$ 1.29	$ 2.23	$ 1.42
Antidilutive dilutive option shares (in shares)	0	0	0
2015 Senior Convertible Debt
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of Convertible Debt (in shares)	4,700,000	13,700,000	9,900,000
2015 Senior Convertible Debt | Weighted Average
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Debt instrument, convertible, conversion price (in dollars per share)	$ 60.90	$ 61.80	$ 62.86
2017 Senior Convertible Debt
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of Convertible Debt (in shares)	1,700,000	100,000	0
2017 Senior Convertible Debt | Weighted Average
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Debt instrument, convertible, conversion price (in dollars per share)	$ 94.96	$ 96.37	$ 98.03
2020 Senior Convertible Debt
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of Convertible Debt (in shares)	0	0	0
2020 Senior Convertible Debt | Weighted Average
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Debt instrument, convertible, conversion price (in dollars per share)	$ 186.86	$ 0	$ 0
2017 Junior Convertible Debt
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of Convertible Debt (in shares)	1,100,000	100,000	0
2017 Junior Convertible Debt | Weighted Average
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Debt instrument, convertible, conversion price (in dollars per share)	$ 93.30	$ 94.68	$ 96.31