PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Components Of Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amortization:
Relating to assets sold during the period	$ 4.8	$ 3.3
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	17.1	17.6	$ 22.7
Interest cost	(30.5)	(30.3)	(37.7)
Expected return on plan assets	(54.5)	(54.7)	(59.8)
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	22.3	21.1	20.8
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement and Curtailment	0.0	0.0	0.2
Amortization:
Amortization of prior service (cost) credit	(2.6)	(2.2)	(2.3)
Net actuarial loss	22.3	21.1	21.0
Net periodic benefit cost	18.0	16.5	23.9
Effect of Curtailment	0.0	0.0	(1.2)
Current year actuarial (gain)/loss	9.3	37.8	(0.7)
Amortization of net loss	22.3	21.1	21.0
Current year prior service cost	0.0	5.7	0.0
Amortization of prior service (cost) credit	(2.6)	(2.2)	(2.3)
Total recognized in other comprehensive income	(15.6)	20.2	(25.2)
Total recognized in net periodic cost and other comprehensive income	2.4	36.7	(1.3)
Defined Benefit Plan, Benefit Obligation	(973.1)	(931.6)	(910.8)
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(223.3)	(245.8)
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1.8	1.7	6.4
Interest cost	(8.3)	(9.1)	(13.4)
Expected return on plan assets	(17.7)	(17.1)	(18.3)
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	4.5	6.0	6.6
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement and Curtailment	0.0	0.0	0.0
Amortization:
Amortization of prior service (cost) credit	3.0	3.7	3.7
Net actuarial loss	4.5	6.0	6.6
Net periodic benefit cost	(6.1)	(4.0)	4.4
Effect of Curtailment	0.0	0.0	0.0
Current year actuarial (gain)/loss	1.2	(8.1)	0.2
Amortization of net loss	4.5	6.0	6.6
Current year prior service cost	0.0	9.8	0.0
Amortization of prior service (cost) credit	3.0	3.7	3.7
Total recognized in other comprehensive income	(0.3)	(0.6)	(2.7)
Total recognized in net periodic cost and other comprehensive income	(6.4)	(4.6)	1.7
Defined Benefit Plan, Benefit Obligation	(265.9)	(264.6)	$ (266.0)
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(3.4)	(11.6)
Salaried Employees [Member] | Pension Benefits [Member]
Amortization:
Defined Benefit Plan, Benefit Obligation	(368.0)	(351.9)
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(98.6)	(109.0)
Salaried Employees [Member] | Other Benefits [Member]
Amortization:
Defined Benefit Plan, Benefit Obligation	(37.7)	(37.6)
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(37.7)	(37.6)
Hourly Employees [Member] | Pension Benefits [Member]
Amortization:
Defined Benefit Plan, Benefit Obligation	(590.0)	(565.6)
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(117.0)	(128.7)
Hourly Employees [Member] | Other Benefits [Member]
Amortization:
Defined Benefit Plan, Benefit Obligation	(228.2)	(227.0)
Defined Benefit Plan, Funded (Unfunded) Status of Plan	34.3	26.0
Mining Employees [Member] | Pension Benefits [Member]
Amortization:
Defined Benefit Plan, Benefit Obligation	(10.3)	(10.0)
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(2.9)	(4.0)
Supplemental Executive Retirement Plan S E R P [Member] | Pension Benefits [Member]
Amortization:
Defined Benefit Plan, Benefit Obligation	(4.8)	(4.1)
Defined Benefit Plan, Funded (Unfunded) Status of Plan	$ (4.8)	$ (4.1)