CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Millions	Total	Common Stock and Additional Paid-in-Capital	Common Stock Held in Treasury	Accumulated Other Comprehensive Loss	Retained Earnings
Balance, common stock shares issued (in shares) at Mar. 31, 2019		253,200,000
Beginning balance at Mar. 31, 2019	$ 5,287.5	$ 2,679.8	$ (582.2)	$ (20.7)	$ 3,210.6
Balance, common stock held in treasury (in shares) at Mar. 31, 2019			15,600,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	50.7				50.7
Other comprehensive income (loss)	(0.3)			(0.3)
Proceeds from sales of common stock through employee equity incentive plans (shares)		500,000
Proceeds from sales of common stock through employee equity incentive plans	7.3	$ 7.3
Restricted stock unit and stock appreciation right withholdings (shares)		(100,000)
Restricted stock unit and stock appreciation right withholdings	(11.4)	$ (11.4)
Treasury stock used for new issuances (shares)		(400,000)	(400,000)
Treasury stock used for new issuances	0.0	$ (13.8)	$ 13.8
Share-based compensation	41.6	$ 41.6
Cash dividend	(87.1)				(87.1)
Balance, common stock shares issued (in shares) at Jun. 30, 2019		253,200,000
Ending balance at Jun. 30, 2019	5,288.3	$ 2,703.5	$ (568.4)	(22.3)	3,175.5
Balance, common stock held in treasury (in shares) at Jun. 30, 2019			15,200,000
Balance, common stock shares issued (in shares) at Mar. 31, 2019		253,200,000
Beginning balance at Mar. 31, 2019	5,287.5	$ 2,679.8	$ (582.2)	(20.7)	3,210.6
Balance, common stock held in treasury (in shares) at Mar. 31, 2019			15,600,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	159.6
Other comprehensive income (loss)	(0.3)
Balance, common stock shares issued (in shares) at Sep. 30, 2019		253,200,000
Ending balance at Sep. 30, 2019	5,361.3	$ 2,731.7	$ (545.2)	(22.3)	3,197.1
Balance, common stock held in treasury (in shares) at Sep. 30, 2019			14,500,000
Balance, common stock shares issued (in shares) at Jun. 30, 2019		253,200,000
Beginning balance at Jun. 30, 2019	5,288.3	$ 2,703.5	$ (568.4)	(22.3)	3,175.5
Balance, common stock held in treasury (in shares) at Jun. 30, 2019			15,200,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	108.9				108.9
Other comprehensive income (loss)	0.0			0.0
Proceeds from sales of common stock through employee equity incentive plans (shares)		900,000
Proceeds from sales of common stock through employee equity incentive plans	20.2	$ 20.2
Restricted stock unit and stock appreciation right withholdings (shares)		(200,000)
Restricted stock unit and stock appreciation right withholdings	(15.2)	$ (15.2)
Treasury stock used for new issuances (shares)		(700,000)	(700,000)
Treasury stock used for new issuances	0.0	$ (23.2)	$ 23.2
Share-based compensation	46.4	$ 46.4
Cash dividend	(87.3)				(87.3)
Balance, common stock shares issued (in shares) at Sep. 30, 2019		253,200,000
Ending balance at Sep. 30, 2019	$ 5,361.3	$ 2,731.7	$ (545.2)	(22.3)	3,197.1
Balance, common stock held in treasury (in shares) at Sep. 30, 2019			14,500,000
Balance, common stock shares issued (in shares) at Mar. 31, 2020	258,391,231	258,400,000
Beginning balance at Mar. 31, 2020	$ 5,585.5	$ 2,675.3	$ (500.6)	(21.6)	3,432.4
Balance, common stock held in treasury (in shares) at Mar. 31, 2020	13,065,588		13,100,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 123.6				123.6
Other comprehensive income (loss)	0.5			0.5
Proceeds from sales of common stock through employee equity incentive plans (shares)		600,000
Proceeds from sales of common stock through employee equity incentive plans	11.0	$ 11.0
Restricted stock unit and stock appreciation right withholdings (shares)		(100,000)
Restricted stock unit and stock appreciation right withholdings	(11.0)	$ (11.0)
Treasury stock used for new issuances (shares)		(500,000)	(500,000)
Treasury stock used for new issuances	0.0	$ (14.8)	$ 14.8
Shares issued to settle convertible debt (in shares)		6,600,000
Shares issued to settle convertible debt	651.5	$ 651.5
Settlement of convertible debt	(810.7)	(810.7)
Share-based compensation	41.3	$ 41.3
Cash dividend	(90.4)				(90.4)
Balance, common stock shares issued (in shares) at Jun. 30, 2020		265,000,000.0
Ending balance at Jun. 30, 2020	$ 5,501.3	$ 2,542.6	$ (485.8)	(21.1)	3,465.6
Balance, common stock held in treasury (in shares) at Jun. 30, 2020			12,600,000
Balance, common stock shares issued (in shares) at Mar. 31, 2020	258,391,231	258,400,000
Beginning balance at Mar. 31, 2020	$ 5,585.5	$ 2,675.3	$ (500.6)	(21.6)	3,432.4
Balance, common stock held in treasury (in shares) at Mar. 31, 2020	13,065,588		13,100,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 197.2
Other comprehensive income (loss)	$ 0.7
Balance, common stock shares issued (in shares) at Sep. 30, 2020	272,255,253	272,300,000
Ending balance at Sep. 30, 2020	$ 5,443.7	$ 2,487.7	$ (467.0)	(20.9)	3,443.9
Balance, common stock held in treasury (in shares) at Sep. 30, 2020	12,000,306		12,000,000.0
Balance, common stock shares issued (in shares) at Jun. 30, 2020		265,000,000.0
Beginning balance at Jun. 30, 2020	$ 5,501.3	$ 2,542.6	$ (485.8)	(21.1)	3,465.6
Balance, common stock held in treasury (in shares) at Jun. 30, 2020			12,600,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	73.6				73.6
Other comprehensive income (loss)	0.2			0.2
Proceeds from sales of common stock through employee equity incentive plans (shares)		700,000
Proceeds from sales of common stock through employee equity incentive plans	18.6	$ 18.6
Restricted stock unit and stock appreciation right withholdings (shares)		(100,000)
Restricted stock unit and stock appreciation right withholdings	(14.4)	$ (14.4)
Treasury stock used for new issuances (shares)		(600,000)	(600,000)
Treasury stock used for new issuances	0.0	$ (18.8)	$ 18.8
Shares issued to settle convertible debt (in shares)		7,300,000
Shares issued to settle convertible debt	768.7	$ 768.7
Settlement of convertible debt	(859.4)	(859.4)
Share-based compensation	50.4	$ 50.4
Cash dividend	$ (95.3)				(95.3)
Balance, common stock shares issued (in shares) at Sep. 30, 2020	272,255,253	272,300,000
Ending balance at Sep. 30, 2020	$ 5,443.7	$ 2,487.7	$ (467.0)	$ (20.9)	$ 3,443.9
Balance, common stock held in treasury (in shares) at Sep. 30, 2020	12,000,306		12,000,000.0