QUARTERLY RESULTS (Unaudited) (Details) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
QUARTERLY RESULTS (Unaudited) [Abstract]
Sales	$ 433.4		$ 240.5		$ 186.6		$ 422.0		$ 387.4		$ 184.7		$ 173.8		$ 383.7		$ 1,282.5		$ 1,129.6		$ 941.9
Gross profit	141.8	[1]	149.8	[1]	37.5	[1]	92.3	[1]	107.0	[1]	45.8	[1]	41.9	[1]	91.3	[1]	421.4		286.0		227.1
Net earnings (loss)	80.5	[1]	87.9	[1]	(0.7)	[1]	50.2	[1]	58.4	[1]	15.4	[1]	10.6	[1]	46.4	[1]	217.9		130.8		88.9
Net earnings (loss) per share, basic (in dollars per share)	$ 2.38		$ 2.60		$ (0.02)		$ 1.49		$ 1.73		$ 0.46		$ 0.32		$ 1.38		$ 6.45		$ 3.89		$ 2.65
Net earnings (loss) per share, diluted (in dollars per share)	$ 2.38		$ 2.60		$ (0.02)		$ 1.49		$ 1.73		$ 0.46		$ 0.32		$ 1.38		$ 6.44		$ 3.88		$ 2.65
Basic weighted-average shares outstanding (in shares)	33,600		33,575		33,549		33,502		33,477		33,469		33,380		33,282		33,557	[2]	33,403	[2]	33,109	[2]
Diluted weighted-average shares outstanding (in shares)	33,617		33,601		33,549		33,520		33,487		33,484		33,411		33,309		33,581		33,420		33,135
Gain from the settlement of an insurance claim			83.3						9.0								83.3		0		0
Gain from the settlement of an insurance claim, net of taxes			60.6						5.7
Gain on product cost			82.3
Gain on selling, general and administrative expenses			1.0
Cost related to refinancing					6.9
Cost related to refinancing, net of tax					5.1
Charges incurred resulting from a court decision									4.7
Charges incurred resulting from a court decision, net of taxes									$ 2.8
8% Senior Notes [Member]
Quarterly Financial Information [Line Items]
Stated interest rate (in hundredths)	8.00%				8.00%												8.00%
4.875% Senior Notes [Member]
Quarterly Financial Information [Line Items]
Stated interest rate (in hundredths)					4.875%

[1] In the third quarter of 2014, the Company recognized a gain of $83.3 million ($60.6 million, net of taxes) from an insurance settlement relating to damage it sustained as a result of a tornado that struck its rock salt mine and evaporation plan in Goderich, Ontario, in 2011. The Company recognized $82.3 million of the gain in product cost and $1.0 million of the gain in selling, general and administrative expenses in the consolidated statements of operations. In the second quarter of 2014, the Company incurred costs of $6.9 million ($5.1 million, net of taxes) related to the refinancing of its 8% Senior Notes with 4.875% Senior Notes. In the fourth quarter of 2013, the Company recognized a gain of $9 million ($5.7 million, net of taxes) from the settlement of an insurance claim resulting from a loss of mineral-concentrated brine due to an asset failure at its solar evaporation ponds in 2010 and a charge of $4.7 million ($2.8 million, net of taxes) from a ruling against the Company related to a labor matter.

[2] For the calculation of diluted earnings per share, the Company uses the more dilutive of either the treasury stock method or the two-class method, to determine the weighted average number of outstanding common shares. In addition, the Company had 381,000, 455,000 and 678,000 weighted options outstanding for 2014, 2013 and 2012, respectively, which were anti-dilutive and therefore not included in the diluted earnings per share calculation.