Net Income Per Common Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended				6 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 73.6	$ 123.6	$ 108.9	$ 50.7	$ 197.2	$ 159.6
Basic weighted average common shares outstanding (in shares)	256,000,000.0		238,400,000		251,900,000	238,100,000
Dilutive effect of stock options and RSUs (in shares)	3,400,000		3,600,000		3,000,000.0	3,600,000
Diluted weighted average common shares outstanding (in shares)	267,700,000		255,300,000		262,800,000	254,600,000
Basic net income per common share (in dollars per share)	$ 0.29		$ 0.46		$ 0.78	$ 0.67
Diluted net income per common share (in dollars per share)	$ 0.27		$ 0.43		$ 0.75	$ 0.63
Antidilutive dilutive option shares (in shares)	0		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)	5,500,000		13,300,000		6,400,000	12,900,000
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)	$ 60.98		$ 61.92		$ 61.10	$ 62.05
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		0		900,000	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)	$ 95.09		$ 96.55		$ 95.27	$ 96.76
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		0		600,000	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)	$ 93.42		$ 94.86		$ 93.60	$ 95.06