Net Income Per Common Share From Continuing Operations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income from continuing operations	$ 174.7	$ 49.2	$ 96.3	$ 35.7	$ 146.7	$ (251.1)	$ 189.2	$ 170.6	$ 355.9	$ 255.4	$ 170.6
Weighted average common shares outstanding (in shares)									236,200,000	232,900,000	217,200,000
Dilutive effect of stock options and RSUs (in shares)									3,800,000	4,400,000	4,400,000
Diluted weighted average common shares outstanding									249,900,000	248,900,000	234,800,000
Antidilutive option shares									0	0	0
Basic net income per common share from continuing operations (in dollars per share)									$ 1.51	$ 1.10	$ 0.79
Diluted net income per common share from continuing operations (in dollars per share)									$ 1.42	$ 1.03	$ 0.73
2007 Junior Convertible Notes
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of subordinated convertible debentures (in shares)									0	1,300,000	12,700,000
Shares issued to settle convertible debt (in shares)										3,700,000	12,000,000.0
Extinguishment of debt, amount										$ 143.8	$ 431.3
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)									9,900,000	10,300,000	500,000
Weighted average conversion price per share used in calculating dilutive effect of convertible debt amount per share (in usd per share)	$ 62.30								$ 62.30
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	0
Weighted average conversion price per share used in calculating dilutive effect of convertible debt amount per share (in usd per share)	97.16								$ 97.16
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	0
Weighted average conversion price per share used in calculating dilutive effect of convertible debt amount per share (in usd per share)	95.45								$ 95.45
Weighted Average | 2007 Junior Convertible Notes
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Weighted average conversion price per share used in calculating dilutive effect of convertible debt amount per share (in usd per share)					$ 23.59					$ 23.59	$ 24.01
Weighted Average | 2015 Senior Convertible Debt
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Weighted average conversion price per share used in calculating dilutive effect of convertible debt amount per share (in usd per share)	62.86				63.94				62.86	63.94	65.21
Weighted Average | 2017 Senior Convertible Debt
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Weighted average conversion price per share used in calculating dilutive effect of convertible debt amount per share (in usd per share)	98.03				99.71				98.03	99.71	100.58
Weighted Average | 2017 Junior Convertible Debt
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Weighted average conversion price per share used in calculating dilutive effect of convertible debt amount per share (in usd per share)	$ 96.31				$ 97.96				$ 96.31	$ 97.96	$ 98.81