EMPLOYEE BENEFIT PLANS - CHANGES IN THE FAIR VALUE OF PLAN ASSETS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	$ 1,160.0	$ 1,168.9
Net realized and unrealized losses	(7.8)	71.2
Ending balance at December 31	1,179.3	1,160.0
Pension Benefits | Level 3
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	9.3
Ending balance at December 31	43.6	9.3
OPEB
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	224.9	222.4
Net realized and unrealized losses	(1.5)	12.0
Ending balance at December 31	216.1	224.9
OPEB | Level 3
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	0.7
Ending balance at December 31	3.0	0.7
Private Placement | Pension Benefits
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	9.3
Ending balance at December 31	43.6	9.3
Private Placement | Pension Benefits | Level 3
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	9.3	0.0
Net realized and unrealized losses	1.4
Purchases	41.8	9.3
Liquidations	(8.9)
Ending balance at December 31	43.6	9.3
Private Placement | OPEB
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	0.7
Ending balance at December 31	3.0	0.7
Private Placement | OPEB | Level 3
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	0.7	0.0
Net realized and unrealized losses	0.1
Purchases	2.8	0.7
Liquidations	(0.6)
Ending balance at December 31	$ 3.0	$ 0.7