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	Accumulated Other Comprehensive Loss Cumulative Effect, Period of Adoption, Adjustment	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment
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
Beginning balance (Accounting Standards Update 2018-02) at Mar. 31, 2019					$ (1.3)		$ 1.3
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
Beginning balance (Accounting Standards Update 2018-02) at Mar. 31, 2019					$ (1.3)		$ 1.3
Balance, common stock held in treasury (in shares) at Mar. 31, 2019			15,600,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	470.7
Other comprehensive income (loss)	0.0
Balance, common stock shares issued (in shares) at Dec. 31, 2019		253,200,000
Ending balance at Dec. 31, 2019	5,616.6	$ 2,738.9	$ (520.8)	(22.0)		3,420.5
Balance, common stock held in treasury (in shares) at Dec. 31, 2019			13,700,000
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	311.1					311.1
Other comprehensive income (loss)	0.3			0.3
Proceeds from sales of common stock through employee equity incentive plans (shares)		1,000,000.0
Proceeds from sales of common stock through employee equity incentive plans	11.3	$ 11.3
Restricted stock unit and stock appreciation right withholdings (shares)		(200,000)
Restricted stock unit and stock appreciation right withholdings	(23.6)	$ (23.6)
Treasury stock used for new issuances (shares)		(800,000)	(800,000)
Treasury stock used for new issuances	0.0	$ (24.4)	$ 24.4
Share-based compensation	43.9	$ 43.9
Cash dividend	(87.7)					(87.7)
Balance, common stock shares issued (in shares) at Dec. 31, 2019		253,200,000
Ending balance at Dec. 31, 2019	$ 5,616.6	$ 2,738.9	$ (520.8)	(22.0)		3,420.5
Balance, common stock held in treasury (in shares) at Dec. 31, 2019			13,700,000
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	$ 233.4
Other comprehensive income (loss)	$ (0.7)
Balance, common stock shares issued (in shares) at Dec. 31, 2020	280,724,898	280,700,000
Ending balance at Dec. 31, 2020	$ 5,297.0	$ 2,385.4	$ (450.2)	(22.3)		3,384.1
Balance, common stock held in treasury (in shares) at Dec. 31, 2020	11,467,154		11,500,000
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		272,300,000
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			12,000,000.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	36.2					36.2
Other comprehensive income (loss)	(1.4)			(1.4)
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	10.6	$ 10.6
Restricted stock unit and stock appreciation right withholdings (shares)		(200,000)
Restricted stock unit and stock appreciation right withholdings	(21.1)	$ (21.1)
Treasury stock used for new issuances (shares)		(500,000)	(500,000)
Treasury stock used for new issuances	0.0	$ (16.8)	$ 16.8
Shares issued to settle convertible debt (in shares)		8,400,000
Shares issued to settle convertible debt	1,160.1	$ 1,160.1
Settlement of convertible debt	(1,339.0)	(1,339.0)
Purchase of capped call options	(35.8)	(35.8)
Issuance of 2020 Senior Convertible Debt	87.7	87.7
Share-based compensation	52.0	$ 52.0
Cash dividend	$ (96.0)					(96.0)
Balance, common stock shares issued (in shares) at Dec. 31, 2020	280,724,898	280,700,000
Ending balance at Dec. 31, 2020	$ 5,297.0	$ 2,385.4	$ (450.2)	$ (22.3)		$ 3,384.1
Balance, common stock held in treasury (in shares) at Dec. 31, 2020	11,467,154		11,500,000