Unaudited Condensed Consolidated Statement Of Shareholders' Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Treasury Stock [Member]
BALANCE at period start (in shares) at Dec. 31, 2018		102,562,149
BALANCE at period start at Dec. 31, 2018	$ 2,716.0	$ 1.0	$ 20.8	$ 3,238.3	$ (544.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	92.0			92.0
Repurchases of common stock	(33.5)				(33.5)
Stock-based compensation expense	13.7		13.7
Shares awarded under stock-based compensation plans, net of shares withheld for taxes	(2.4)		(9.8)		7.4
BALANCE at period end (in shares) at Mar. 31, 2019		102,562,149
BALANCE at period end at Mar. 31, 2019	2,785.8	$ 1.0	24.7	3,330.3	(570.2)
BALANCE at period start (in shares) at Dec. 31, 2018		102,562,149
BALANCE at period start at Dec. 31, 2018	2,716.0	$ 1.0	20.8	3,238.3	(544.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	192.8
BALANCE at period end (in shares) at Jun. 30, 2019		102,562,149
BALANCE at period end at Jun. 30, 2019	2,882.9	$ 1.0	25.6	3,431.1	(574.8)
BALANCE at period start (in shares) at Mar. 31, 2019		102,562,149
BALANCE at period start at Mar. 31, 2019	2,785.8	$ 1.0	24.7	3,330.3	(570.2)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	100.8			100.8
Repurchases of common stock	(11.2)				(11.2)
Stock-based compensation expense	4.9		4.9
Shares awarded under stock-based compensation plans, net of shares withheld for taxes	2.6		(4.0)		6.6
BALANCE at period end (in shares) at Jun. 30, 2019		102,562,149
BALANCE at period end at Jun. 30, 2019	2,882.9	$ 1.0	25.6	3,431.1	(574.8)
BALANCE at period start (in shares) at Dec. 31, 2019		102,562,149
BALANCE at period start at Dec. 31, 2019	3,162.1	$ 1.0	35.9	3,688.3	(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	$ 1.0	35.9	3,688.3	(563.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	47.5
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)
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)