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Net Income Per Common Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 99.9	$ 311.1	$ 108.9	$ 50.7	$ 174.7	$ 49.2	$ 96.3	$ 35.7	$ 570.6	$ 355.9	$ 255.4
Basic weighted average common shares outstanding (in shares)									238,900,000	236,200,000	232,900,000
Dilutive effect of stock options and RSUs (in shares)									3,400,000	3,800,000	4,400,000
Diluted weighted average common shares outstanding (in shares)									256,200,000	249,900,000	248,900,000
Basic net income per common share (in dollars per share)									$ 2.39	$ 1.51	$ 1.10
Diluted net income per common share (in dollars per share)	$ 0.39	$ 1.20	$ 0.43	$ 0.20	$ 0.70	$ 0.20	$ 0.38	$ 0.14	$ 2.23	$ 1.42	$ 1.03
Antidilutive dilutive option shares (in shares)									0	0	0
2007 Junior Convertible Notes
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of Convertible Debt (in shares)									0	0	1,300,000
2007 Junior Convertible Notes \| Weighted Average
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Debt instrument, convertible, conversion price (in dollars per share)											$ 23.59
2015 Senior Convertible Debt
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of Convertible Debt (in shares)									13,700,000	9,900,000	10,300,000
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)	61.80				62.86				$ 61.80	$ 62.86	$ 63.94
2017 Senior Convertible Debt
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of Convertible Debt (in shares)									100,000	0	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)	96.37				98.03				$ 96.37	$ 98.03	$ 99.71
2017 Junior Convertible Debt
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of Convertible Debt (in shares)									100,000	0	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)	$ 94.68				$ 96.31				$ 94.68	$ 96.31	$ 97.96