Unaudited Condensed Consolidated Statement Of Shareholders' Equity - USD ($) $ in Millions	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Retained Earnings [Member] Cumulative Effect, Period of Adoption, Adjustment	Treasury Stock [Member]
BALANCE at period start (in shares) at Dec. 31, 2019			102,562,149
BALANCE at period start at Dec. 31, 2019	$ 3,162.1	$ (0.5)	$ 1.0	$ 35.9	$ 3,688.3	$ (0.5)	$ (563.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(232.3)				(232.3)
Repurchases of common stock	(80.0)						(80.0)
Stock-based compensation expense	4.5			4.5
Shares awarded under stock-based compensation plans, net of shares withheld for taxes	(6.9)			(21.7)			14.8
BALANCE at period end (in shares) at Mar. 31, 2020			102,562,149
BALANCE at period end at Mar. 31, 2020	2,846.9		$ 1.0	18.7	3,455.5		(628.3)
BALANCE at period start (in shares) at Dec. 31, 2019			102,562,149
BALANCE at period start at Dec. 31, 2019	3,162.1	$ (0.5)	$ 1.0	35.9	3,688.3	$ (0.5)	(563.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	230.1
BALANCE at period end (in shares) at Sep. 30, 2020			102,562,149
BALANCE at period end at Sep. 30, 2020	3,357.9		$ 1.0	41.3	3,917.9		(602.3)
BALANCE at period start (in shares) at Mar. 31, 2020			102,562,149
BALANCE at period start at Mar. 31, 2020	2,846.9		$ 1.0	18.7	3,455.5		(628.3)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	279.8				279.8
Stock-based compensation expense	11.2			11.2
Shares awarded under stock-based compensation plans, net of shares withheld for taxes	(0.2)			(0.4)			0.2
BALANCE at period end (in shares) at Jun. 30, 2020			102,562,149
BALANCE at period end at Jun. 30, 2020	3,137.7		$ 1.0	29.5	3,735.3		(628.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	182.6				182.6
Stock-based compensation expense	7.2			7.2
Shares awarded under stock-based compensation plans, net of shares withheld for taxes	30.4			4.6			25.8
BALANCE at period end (in shares) at Sep. 30, 2020			102,562,149
BALANCE at period end at Sep. 30, 2020	3,357.9		$ 1.0	41.3	3,917.9		(602.3)
BALANCE at period start (in shares) at Dec. 31, 2020			102,562,149
BALANCE at period start at Dec. 31, 2020	3,235.7		$ 1.0	53.1	4,069.4		(887.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	239.4				239.4
Repurchases of common stock	(306.1)						(306.1)
Stock-based compensation expense	20.8			20.8
Shares awarded under stock-based compensation plans, net of shares withheld for taxes	4.1			(32.9)			37.0
BALANCE at period end (in shares) at Mar. 31, 2021			102,562,149
BALANCE at period end at Mar. 31, 2021	3,193.9		$ 1.0	41.0	4,308.8		(1,156.9)
BALANCE at period start (in shares) at Dec. 31, 2020			102,562,149
BALANCE at period start at Dec. 31, 2020	3,235.7		$ 1.0	53.1	4,069.4		(887.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	985.9
BALANCE at period end (in shares) at Sep. 30, 2021			86,562,149
BALANCE at period end at Sep. 30, 2021	2,356.5		$ 0.8	1.6	4,258.1		(1,904.0)
BALANCE at period start (in shares) at Mar. 31, 2021			102,562,149
BALANCE at period start at Mar. 31, 2021	3,193.9		$ 1.0	41.0	4,308.8		(1,156.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	384.8				384.8
Repurchases of common stock	(736.1)						(736.1)
Treasury stock cancellation, shares			(16,000,000)
Treasury stock cancellation	0.0		$ (0.2)	(40.6)	(797.2)		838.0
Stock-based compensation expense	6.5			6.5
Shares awarded under stock-based compensation plans, net of shares withheld for taxes	7.7			(1.2)			8.9
BALANCE at period end (in shares) at Jun. 30, 2021			86,562,149
BALANCE at period end at Jun. 30, 2021	2,856.8		$ 0.8	5.7	3,896.4		(1,046.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	361.7				361.7
Repurchases of common stock	(879.2)						(879.2)
Stock-based compensation expense	4.3			4.3
Shares awarded under stock-based compensation plans, net of shares withheld for taxes	12.9			(8.4)			21.3
BALANCE at period end (in shares) at Sep. 30, 2021			86,562,149
BALANCE at period end at Sep. 30, 2021	$ 2,356.5		$ 0.8	$ 1.6	$ 4,258.1		$ (1,904.0)