ACCUMULATED OTHER COMPREHENSIVE LOSS (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Changes in accumulated other comprehensive loss by component, net of taxes
Balance at the beginning of the period			$ 494.9				$ 494.9
Net gain in prior service cost	$ 0.7			$ 23.4			0.8	$ 24.1
Amortization of prior service cost	(2.2)			(0.6)			(2.9)	(1.2)
Amortization of net loss (gain)	1.0			0.9			3.1	3.0
Other adjustments				0.6			0.2	0.5
Balance at the end of the period	518.6						518.6		$ 494.9
Pensions
Changes in accumulated other comprehensive loss by component, net of taxes
Balance at the beginning of the period	(40.5)	$ (41.0)	(41.4)	(40.8)	$ (41.3)	$ (41.7)	(41.4)	(41.7)	(41.7)
Amortization of prior service cost	(0.4)	(0.3)	(0.4)	(0.3)	(0.3)	(0.4)
Amortization of net loss (gain)	0.8	0.8	0.8	0.8	0.8	0.8
Balance at the end of the period	(40.1)	(40.5)	(41.0)	(40.3)	(40.8)	(41.3)	(40.1)	(40.3)	(41.4)
Post Retirement
Changes in accumulated other comprehensive loss by component, net of taxes
Balance at the beginning of the period	18.5	18.2	18.1	(3.6)	(3.8)	(4.7)	18.1	(4.7)	(4.7)
Net gain in prior service cost	0.7			23.4		0.7
Amortization of prior service cost	(1.7)			(0.3)
Amortization of net loss (gain)	0.2	0.3	0.1	0.2	0.2	0.2
Balance at the end of the period	17.7	18.5	18.2	19.7	(3.6)	(3.8)	17.7	19.7	18.1
Non-Qualified Plans
Changes in accumulated other comprehensive loss by component, net of taxes
Balance at the beginning of the period	(0.3)	(0.4)	(0.4)		(0.1)	(0.2)	(0.4)	(0.2)	(0.2)
Amortization of prior service cost	(0.1)				0.1
Amortization of net loss (gain)		0.1		(0.1)		0.1
Balance at the end of the period	(0.4)	(0.3)	(0.4)	(0.1)		(0.1)	(0.4)	(0.1)	(0.4)
Other
Changes in accumulated other comprehensive loss by component, net of taxes
Balance at the beginning of the period	0.4	0.3	0.1	(0.4)	(0.4)	(0.3)	0.1	(0.3)	(0.3)
Net gain in prior service cost		0.1
Other adjustments			0.2	0.6		(0.1)
Balance at the end of the period	0.4	0.4	0.3	0.2	(0.4)	(0.4)	0.4	0.2	0.1
Accumulated Other Comprehensive Income (Loss)
Changes in accumulated other comprehensive loss by component, net of taxes
Balance at the beginning of the period	(21.9)	(22.9)	(23.6)	(44.8)	(45.6)	(46.9)	(23.6)	(46.9)	(46.9)
Net gain in prior service cost	0.7	0.1		23.4		0.7
Amortization of prior service cost	(2.2)	(0.3)	(0.4)	(0.6)	(0.2)	(0.4)
Amortization of net loss (gain)	1.0	1.2	0.9	0.9	1.0	1.1
Other adjustments			0.2	0.6		(0.1)
Balance at the end of the period	$ (22.4)	$ (21.9)	$ (22.9)	$ (20.5)	$ (44.8)	$ (45.6)	$ (22.4)	$ (20.5)	$ (23.6)