Net Income Per Common Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended				6 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 108.9	$ 50.7	$ 96.3	$ 35.7	$ 159.6	$ 132.0
Basic weighted average common shares outstanding (in shares)	238,400,000		235,800,000		238,100,000	235,500,000
Dilutive effect of stock options and RSUs	3,600,000		4,300,000		3,600,000	4,200,000
Diluted weighted average common shares outstanding (in shares)	255,300,000		251,800,000		254,600,000	252,000,000.0
Basic net income per common share (in dollars per share)	$ 0.46		$ 0.41		$ 0.67	$ 0.56
Diluted net income per common share (in dollars per share)	$ 0.43		$ 0.38		$ 0.63	$ 0.52
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	13,300,000		11,700,000		12,900,000	12,300,000
2015 Senior Convertible Debt | Weighted Average
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Conversion price per share used in calculating dilutive effect of convertible debt amount per share (in dollars per share)	$ 61.92		$ 63.02		$ 62.05	$ 63.14
2017 Senior Convertible Debt
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of Convertible Debt	0		0		0	0
2017 Senior Convertible Debt | Weighted Average
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Conversion price per share used in calculating dilutive effect of convertible debt amount per share (in dollars per share)	$ 96.55		$ 98.27		$ 96.76	$ 98.47
2017 Junior Convertible Debt
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of Convertible Debt	0		0		0	0
2017 Junior Convertible Debt | Weighted Average
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Conversion price per share used in calculating dilutive effect of convertible debt amount per share (in dollars per share)	$ 94.86		$ 96.55		$ 95.06	$ 96.74