SIGNIFICANT ACCOUNTING POLICIES - COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Changes in accumulated other comprehensive loss by component, net of taxes
Balance	$ 363.8	$ 338.2	$ 279.9
Net gain (loss) in prior service cost	5.1	(31.4)	18.7
Amortization of prior service cost	(1.3)	(1.3)	(1.3)
Amortization of net loss included in net periodic pension cost	1.8	2.5	4.7
Foreign currency translation adjustment	0.7	0.4	(0.1)
Other	0.1
Balance	450.6	363.8	338.2
Comprehensive Income (Loss):
Tax benefit (expense) on other comprehensive income (loss)	(5.0)	19.4	(14.1)
Qualified Pension Benefits
Changes in accumulated other comprehensive loss by component, net of taxes
Balance	(45.0)	(20.1)
Net gain (loss) in prior service cost	3.7	(25.4)
Amortization of prior service cost	(1.3)	(1.3)
Amortization of net loss included in net periodic pension cost	0.9	1.8
Balance	(41.7)	(45.0)	(20.1)
Post-retirement Benefits
Changes in accumulated other comprehensive loss by component, net of taxes
Balance	(7.2)	(1.7)
Net gain (loss) in prior service cost	1.4	(5.9)
Amortization of prior service cost	0.1
Amortization of net loss included in net periodic pension cost	1.0	0.4
Balance	(4.7)	(7.2)	(1.7)
Non-qualified Pension Benefits
Changes in accumulated other comprehensive loss by component, net of taxes
Balance	(0.7)	(0.9)
Net gain (loss) in prior service cost		(0.1)
Amortization of prior service cost	(0.1)
Amortization of net loss included in net periodic pension cost	(0.1)	0.3
Balance	(0.9)	(0.7)	(0.9)
Foreign Currency Translation
Changes in accumulated other comprehensive loss by component, net of taxes
Balance	0.3	(0.1)
Foreign currency translation adjustment	0.7	0.4
Balance	1.0	0.3	(0.1)
Interest Hedge
Changes in accumulated other comprehensive loss by component, net of taxes
Balance	(0.7)	(0.7)
Other	0.1
Balance	(0.6)	(0.7)	(0.7)
Accumulated Other Comprehensive Income (Loss)
Changes in accumulated other comprehensive loss by component, net of taxes
Balance	(53.3)	(23.5)	(45.5)
Net gain (loss) in prior service cost	5.1	(31.4)
Amortization of prior service cost	(1.3)	(1.3)
Amortization of net loss included in net periodic pension cost	1.8	2.5
Foreign currency translation adjustment	0.7	0.4
Other	0.1
Balance	$ (46.9)	$ (53.3)	$ (23.5)