PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2016	Dec. 31, 2016	Aug. 31, 2016
Pension and SERP [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Service cost discount rate under spot rate		4.89%
Interest cost discount rate under spot rate methodology		3.80%
Impact of change in accounting estimate spot rate discount rate	$ 11.5	$ 35.0
Other Postretirement Benefit Plan [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Service cost discount rate under spot rate		4.09%
Interest cost discount rate under spot rate methodology		2.88%
Impact of change in accounting estimate spot rate discount rate	$ 2.5	$ 7.5
Discount rate used to remeasure benefit obligation				3.62%
Gross impact of plan amendment on accumulated losses				$ 244.0
Actuarial Loss				142.0
Effect of plan amendment on accumulated benefit obligation				102.0
Effect of plan amendment on regulatory assets				106.0
Effect of plan amendment on accumulated other comprehensive income				$ 4.0
Subsequent Event [Member] | Other Postretirement Benefit Plan [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Impact on annual PBOP expense			$ (10.0)