PENSION AND POST-RETIREMENT PLANS - FAIR VALUE (Details) - Pension Benefits - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension and Post-retirement Plans
Total	$ 186.7	$ 178.8	$ 168.9
Significant Unobservable Inputs (Level 3) | Private equity partnership interests
Pension and Post-retirement Plans
Total	0.1	0.1	$ 0.2
Fair Value Measurement
Pension and Post-retirement Plans
Total	186.7	178.8
Fair Value Measurement | Cash
Pension and Post-retirement Plans
Total	6.6	7.8
Fair Value Measurement | U.S. large-cap
Pension and Post-retirement Plans
Total	66.0	67.1
Fair Value Measurement | U.S. mid- and small-cap
Pension and Post-retirement Plans
Total	42.6	38.9
Fair Value Measurement | International large-cap
Pension and Post-retirement Plans
Total	21.6	16.9
Fair Value Measurement | International small-cap
Pension and Post-retirement Plans
Total	9.5	6.9
Fair Value Measurement | U.S. Treasuries
Pension and Post-retirement Plans
Total	8.0	8.9
Fair Value Measurement | Municipal bonds
Pension and Post-retirement Plans
Total	0.1	9.3
Fair Value Measurement | Investment grade U.S. corporate bonds
Pension and Post-retirement Plans
Total	17.5	5.3
Fair Value Measurement | High-yield U.S. corporate bonds
Pension and Post-retirement Plans
Total	3.6	6.3
Fair Value Measurement | Emerging markets fixed income
Pension and Post-retirement Plans
Total		0.5
Fair Value Measurement | Real estate partnerships interests
Pension and Post-retirement Plans
Total	11.1	10.8
Fair Value Measurement | Private equity partnership interests
Pension and Post-retirement Plans
Total	0.1	0.1
Fair Value Measurement | Quoted Prices in Active Markets (Level 1)
Pension and Post-retirement Plans
Total	63.0	71.6
Fair Value Measurement | Quoted Prices in Active Markets (Level 1) | Cash
Pension and Post-retirement Plans
Total	6.6	7.8
Fair Value Measurement | Quoted Prices in Active Markets (Level 1) | U.S. large-cap
Pension and Post-retirement Plans
Total	28.1	31.6
Fair Value Measurement | Quoted Prices in Active Markets (Level 1) | U.S. mid- and small-cap
Pension and Post-retirement Plans
Total	28.3	31.7
Fair Value Measurement | Quoted Prices in Active Markets (Level 1) | Emerging markets fixed income
Pension and Post-retirement Plans
Total		0.5
Fair Value Measurement | Significant Observable Inputs (Level 2)
Pension and Post-retirement Plans
Total	112.5	96.3
Fair Value Measurement | Significant Observable Inputs (Level 2) | U.S. large-cap
Pension and Post-retirement Plans
Total	37.9	35.5
Fair Value Measurement | Significant Observable Inputs (Level 2) | U.S. mid- and small-cap
Pension and Post-retirement Plans
Total	14.3	7.2
Fair Value Measurement | Significant Observable Inputs (Level 2) | International large-cap
Pension and Post-retirement Plans
Total	21.6	16.9
Fair Value Measurement | Significant Observable Inputs (Level 2) | International small-cap
Pension and Post-retirement Plans
Total	9.5	6.9
Fair Value Measurement | Significant Observable Inputs (Level 2) | U.S. Treasuries
Pension and Post-retirement Plans
Total	8.0	8.9
Fair Value Measurement | Significant Observable Inputs (Level 2) | Municipal bonds
Pension and Post-retirement Plans
Total	0.1	9.3
Fair Value Measurement | Significant Observable Inputs (Level 2) | Investment grade U.S. corporate bonds
Pension and Post-retirement Plans
Total	17.5	5.3
Fair Value Measurement | Significant Observable Inputs (Level 2) | High-yield U.S. corporate bonds
Pension and Post-retirement Plans
Total	3.6	6.3
Fair Value Measurement | Significant Unobservable Inputs (Level 3)
Pension and Post-retirement Plans
Total	11.2	10.9
Fair Value Measurement | Significant Unobservable Inputs (Level 3) | Real estate partnerships interests
Pension and Post-retirement Plans
Total	11.1	10.8
Fair Value Measurement | Significant Unobservable Inputs (Level 3) | Private equity partnership interests
Pension and Post-retirement Plans
Total	$ 0.1	$ 0.1