Condensed Statements of Shareholders' Equity - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Loss	Retained Earnings
Balance (in shares) at Dec. 31, 2017		167.1
Balance at Dec. 31, 2017	$ 2,753.7	$ 167.1	$ 2,280.9	$ (484.6)	$ 790.3
Income tax reclassification adjustment	0.0	0.0	0.0	(85.9)	85.9
Net income (loss)	20.9	0.0	0.0	0.0	20.9
Other comprehensive (loss) income	17.7	0.0	0.0	17.7	0.0
Dividends paid:
Common stock ($0.20 per share)	(33.4)	$ 0.0	0.0	0.0	(33.4)
Common stock issued for:
Stock options exercised (in shares)		0.1
Stock options exercised	1.0	$ 0.1	0.9	0.0	0.0
Other transactions (in shares)		0.0
Other transactions	0.5	$ 0.0	0.5	0.0	0.0
Stock-based compensation	2.7	$ 0.0	2.7	0.0	0.0
Balance (in shares) at Mar. 31, 2018		167.2
Balance at Mar. 31, 2018	2,763.1	$ 167.2	2,285.0	(552.8)	863.7
Balance (in shares) at Dec. 31, 2017		167.1
Balance at Dec. 31, 2017	2,753.7	$ 167.1	2,280.9	(484.6)	790.3
Net income (loss)	274.6
Other comprehensive (loss) income	$ (0.8)
Common stock issued for:
Stock options exercised (in shares)	0.2
Stock options exercised	$ 3.2
Balance (in shares) at Sep. 30, 2018		166.8
Balance at Sep. 30, 2018	2,922.9	$ 166.8	2,276.9	(571.3)	1,050.5
Balance (in shares) at Mar. 31, 2018		167.2
Balance at Mar. 31, 2018	2,763.1	$ 167.2	2,285.0	(552.8)	863.7
Net income (loss)	58.6	0.0	0.0	0.0	58.6
Other comprehensive (loss) income	(20.4)	0.0	0.0	(20.4)	0.0
Dividends paid:
Common stock ($0.20 per share)	(33.5)	$ 0.0	0.0	0.0	(33.5)
Common stock repurchased and retired (in shares)		(0.3)
Common stock repurchased and retired	(9.1)	$ (0.3)	(8.8)	0.0	0.0
Common stock issued for:
Stock options exercised (in shares)		0.1
Stock options exercised	1.2	$ 0.1	1.1	0.0	0.0
Other transactions (in shares)		0.0
Other transactions	0.1	$ 0.0	0.1	0.0	0.0
Stock-based compensation	3.1	$ 0.0	3.1	0.0	0.0
Balance (in shares) at Jun. 30, 2018		167.0
Balance at Jun. 30, 2018	2,763.1	$ 167.0	2,280.5	(573.2)	888.8
Net income (loss)	195.1	0.0	0.0	0.0	195.1
Other comprehensive (loss) income	1.9	0.0	0.0	1.9	0.0
Dividends paid:
Common stock ($0.20 per share)	(33.4)	$ 0.0	0.0	0.0	(33.4)
Common stock repurchased and retired (in shares)		(0.2)
Common stock repurchased and retired	(7.7)	$ (0.2)	(7.5)	0.0	0.0
Common stock issued for:
Stock options exercised (in shares)		0.0
Stock options exercised	1.0	$ 0.0	1.0	0.0	0.0
Other transactions (in shares)		0.0
Other transactions	0.9	$ 0.0	0.9	0.0	0.0
Stock-based compensation	2.0	$ 0.0	2.0	0.0	0.0
Balance (in shares) at Sep. 30, 2018		166.8
Balance at Sep. 30, 2018	2,922.9	$ 166.8	2,276.9	(571.3)	1,050.5
Balance (in shares) at Dec. 31, 2018		165.3
Balance at Dec. 31, 2018	2,832.2	$ 165.3	2,247.4	(651.0)	1,070.5
Lease accounting adoption adjustment	11.2	0.0	0.0	0.0	11.2
Net income (loss)	41.7	0.0	0.0	0.0	41.7
Other comprehensive (loss) income	(5.9)	0.0	0.0	(5.9)	0.0
Dividends paid:
Common stock ($0.20 per share)	(33.0)	$ 0.0	0.0	0.0	(33.0)
Common stock repurchased and retired (in shares)		(0.6)
Common stock repurchased and retired	(13.2)	$ (0.6)	(12.6)	0.0	0.0
Common stock issued for:
Stock options exercised (in shares)		0.1
Stock options exercised	1.4	$ 0.1	1.3	0.0	0.0
Other transactions (in shares)		0.1
Other transactions	0.4	$ 0.1	0.3	0.0	0.0
Stock-based compensation	2.8	$ 0.0	2.8	0.0	0.0
Balance (in shares) at Mar. 31, 2019		164.9
Balance at Mar. 31, 2019	2,837.6	$ 164.9	2,239.2	(656.9)	1,090.4
Balance (in shares) at Dec. 31, 2018		165.3
Balance at Dec. 31, 2018	2,832.2	$ 165.3	2,247.4	(651.0)	1,070.5
Net income (loss)	65.9
Other comprehensive (loss) income	$ (21.7)
Common stock issued for:
Stock options exercised (in shares)	0.1
Stock options exercised	$ 1.5
Balance (in shares) at Sep. 30, 2019		159.7
Balance at Sep. 30, 2019	2,664.7	$ 159.7	2,128.6	(672.7)	1,049.1
Balance (in shares) at Mar. 31, 2019		164.9
Balance at Mar. 31, 2019	2,837.6	$ 164.9	2,239.2	(656.9)	1,090.4
Net income (loss)	(20.0)	0.0	0.0	0.0	(20.0)
Other comprehensive (loss) income	(3.5)	0.0	0.0	(3.5)	0.0
Dividends paid:
Common stock ($0.20 per share)	(32.7)	$ 0.0	0.0	0.0	(32.7)
Common stock repurchased and retired (in shares)		(0.6)
Common stock repurchased and retired	(13.5)	$ (0.6)	(12.9)	0.0	0.0
Common stock issued for:
Stock options exercised (in shares)		0.0
Stock options exercised	0.1	$ 0.0	0.1	0.0	0.0
Other transactions (in shares)		0.0
Other transactions	0.9	$ 0.0	0.9	0.0	0.0
Stock-based compensation	1.9	$ 0.0	1.9	0.0	0.0
Balance (in shares) at Jun. 30, 2019		164.3
Balance at Jun. 30, 2019	2,770.8	$ 164.3	2,229.2	(660.4)	1,037.7
Net income (loss)	44.2	0.0	0.0	0.0	44.2
Other comprehensive (loss) income	(12.3)	0.0	0.0	(12.3)	0.0
Dividends paid:
Common stock ($0.20 per share)	(32.8)	$ 0.0	0.0	0.0	(32.8)
Common stock repurchased and retired (in shares)		(4.8)
Common stock repurchased and retired	(108.9)	$ (4.8)	(104.1)	0.0	0.0
Common stock issued for:
Stock options exercised (in shares)		0.0
Stock options exercised	0.0	$ 0.0	0.0	0.0	0.0
Other transactions (in shares)		0.2
Other transactions	1.8	$ 0.2	1.6	0.0	0.0
Stock-based compensation	1.9	$ 0.0	1.9	0.0	0.0
Balance (in shares) at Sep. 30, 2019		159.7
Balance at Sep. 30, 2019	$ 2,664.7	$ 159.7	$ 2,128.6	$ (672.7)	$ 1,049.1