Net Income Per Common Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2019
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 50.7	$ 35.7
Basic common shares outstanding (in shares)	237,800,000	235,200,000
Dilutive effect of stock options and RSUs (in shares)	3,700,000	4,000,000.0
Diluted weighted average common shares outstanding (in shares)	253,900,000	252,200,000
Basic net income per common share (in dollars per share)	$ 0.21	$ 0.15
Diluted net income per common share (in dollars per share)	$ 0.20	$ 0.14
Antidilutive dilutive option shares (in shares)	0	0
Other assets	$ 232.3		$ 111.8
2015 Senior Convertible Debt
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of subordinated convertible debentures (in shares)	12,400,000	13,000,000.0
Conversion price per share used in calculating dilutive effect of convertible debt amount per share (in dollars per share)	$ 62.03
2017 Senior Convertible Debt
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of subordinated convertible debentures (in shares)	0	0
Conversion price per share used in calculating dilutive effect of convertible debt amount per share (in dollars per share)	$ 96.73
2017 Junior Convertible Debt
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of subordinated convertible debentures (in shares)	0	0
Conversion price per share used in calculating dilutive effect of convertible debt amount per share (in dollars per share)	$ 95.03
Weighted Average | 2015 Senior Convertible Debt
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)	62.18	$ 63.27
Weighted Average | 2017 Senior Convertible Debt
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)	96.96	98.67
Weighted Average | 2017 Junior Convertible Debt
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)	$ 95.26	$ 96.93