EARNINGS PER-SHARE (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Income
Net Income, Basic									$ 232.0	$ 81.4	$ 103.0
Net Income, Diluted									$ 232.0	$ 81.4	$ 103.0
Weighted Average Common Shares
Basic (in shares)									42.9	43.1	43.5
Effect of Dilutive Securities (in shares)									0.3	0.4	0.5
Diluted (in shares)									43.2	43.5	44.0
Per Common Share Amount
Net income, Basic (in dollars per share)	$ 3.93	$ 0.79	$ 0.56	$ 0.16	$ 0.47	$ 0.59	$ 0.42	$ 0.42	$ 5.41	$ 1.89	$ 2.37
Effect of Dilutive Securities (in dollars per shares)									(0.04)	(0.02)	(0.03)
Net income, Diluted (in dollars per share)	$ 3.90	$ 0.79	$ 0.55	$ 0.16	$ 0.46	$ 0.59	$ 0.42	$ 0.41	$ 5.37	$ 1.87	$ 2.34