Net Income Per Common Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended						9 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2020	Dec. 31, 2019
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 36.2	$ 73.6	$ 123.6	$ 311.1	$ 108.9	$ 50.7	$ 233.4	$ 470.7
Basic weighted average common shares outstanding (in shares)	263,400,000			239,200,000			255,700,000	238,500,000
Dilutive effect of stock options and RSUs (in shares)	3,700,000			3,400,000			3,400,000	3,500,000
Diluted weighted average common shares outstanding (in shares)	275,400,000			258,300,000			267,000,000.0	255,800,000
Basic net income per common share (in dollars per share)	$ 0.14			$ 1.30			$ 0.91	$ 1.97
Diluted net income per common share (in dollars per share)	$ 0.13			$ 1.20			$ 0.87	$ 1.84
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)	3,500,000			15,200,000			5,400,000	13,600,000
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)	$ 60.78			$ 61.67			$ 60.99	$ 61.92
2017 Senior Convertible Debt
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of Convertible Debt (in shares)	2,800,000			300,000			1,500,000	100,000
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)	$ 94.78			$ 96.16			$ 95.11	$ 96.56
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	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)	$ 186.87			$ 0			$ 186.87	$ 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)	2,000,000.0			200,000			1,000,000.0	100,000
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)	$ 93.12			$ 94.48			$ 93.44	$ 94.87