ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in accumulated other comprehensive loss by component, net of taxes
Balance at the beginning of the period	$ 494.9	$ 450.6	$ 363.8
Net gain (loss) in prior service cost	0.8	24.1	5.1
Amortization of prior service cost	(4.0)	(2.2)	(1.3)
Amortization of net loss (gain)	1.7	1.2	1.8
Other adjustments	0.2	0.2	0.8
Balance at the end of the period	678.2	494.9	450.6
Comprehensive Income (Loss):
Tax benefit (expense) on other comprehensive income (loss)	(4.4)	(14.2)	(5.0)
Pensions
Changes in accumulated other comprehensive loss by component, net of taxes
Balance at the beginning of the period	(41.4)	(41.7)
Amortization of prior service cost	(1.4)	(1.4)
Amortization of net loss (gain)	2.2	1.7
Balance at the end of the period	(40.6)	(41.4)	(41.7)
Post Retirement
Changes in accumulated other comprehensive loss by component, net of taxes
Balance at the beginning of the period	18.1	(4.7)
Net gain (loss) in prior service cost	0.7	24.1
Amortization of prior service cost	(2.5)	(0.9)
Amortization of net loss (gain)	(0.7)	(0.4)
Balance at the end of the period	15.6	18.1	(4.7)
Non-Qualified Plans
Changes in accumulated other comprehensive loss by component, net of taxes
Balance at the beginning of the period	(0.4)	(0.2)
Net gain (loss) in prior service cost		(0.1)
Amortization of prior service cost	(0.1)	(0.1)
Amortization of net loss (gain)	0.2
Balance at the end of the period	(0.3)	(0.4)	(0.2)
Other
Changes in accumulated other comprehensive loss by component, net of taxes
Balance at the beginning of the period	0.1	(0.3)
Net gain (loss) in prior service cost	0.1	0.1
Amortization of prior service cost		0.2
Amortization of net loss (gain)		(0.1)
Other adjustments	0.2	0.2
Balance at the end of the period	0.4	0.1	(0.3)
Accumulated Other Comprehensive Income (Loss)
Changes in accumulated other comprehensive loss by component, net of taxes
Balance at the beginning of the period	(23.6)	(46.9)	(53.3)
Net gain (loss) in prior service cost	0.8	24.1
Amortization of prior service cost	(4.0)	(2.2)
Amortization of net loss (gain)	1.7	1.2
Other adjustments	0.2	0.2
Balance at the end of the period	$ (24.9)	$ (23.6)	$ (46.9)