PENSION AND OTHER POSTRETIREMENT BENEFITS - VEBA (Effect of Fair Value Measurements Using Significant Unobservable Inputs (Level 3) on Changes in Other Benefit Plan Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	$ 240.2	$ 262.5
Actual return on plan assets:
Fair value of plan assets — end of year	259.7	240.2
Other Postretirement Benefits Plan [Member] | Hedge Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	11.4
Actual return on plan assets:
Fair value of plan assets — end of year	12.4	11.4
Other Postretirement Benefits Plan [Member] | Private Equity Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	3.0
Actual return on plan assets:
Fair value of plan assets — end of year	1.7	3.0
Other Postretirement Benefits Plan [Member] | Structured Credit [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	8.5
Actual return on plan assets:
Fair value of plan assets — end of year	5.5	8.5
Other Postretirement Benefits Plan [Member] | Real Estate [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	13.1
Actual return on plan assets:
Fair value of plan assets — end of year	14.0	13.1
Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	229.1	215.6
Actual return on plan assets:
Relating to assets still held at the reporting date	(1.3)	12.5
Relating to assets sold during the period	30.6	4.0
Purchases	16.5	5.2
Sales	(59.9)	(8.2)
Fair value of plan assets — end of year	212.4	229.1
Significant Unobservable Inputs (Level 3) [Member] | Hedge Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	37.2	37.4
Actual return on plan assets:
Relating to assets still held at the reporting date	3.4	(0.2)
Relating to assets sold during the period	0.0	0.0
Purchases	0.0	0.0
Sales	0.0	0.0
Fair value of plan assets — end of year	40.2	37.2
Significant Unobservable Inputs (Level 3) [Member] | Private Equity Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	42.6	39.8
Actual return on plan assets:
Relating to assets still held at the reporting date	(1.0)	1.4
Relating to assets sold during the period	1.2	4.0
Purchases	16.5	5.2
Sales	(9.3)	(7.8)
Fair value of plan assets — end of year	49.5	42.6
Significant Unobservable Inputs (Level 3) [Member] | Real Estate [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	70.5	65.5
Actual return on plan assets:
Relating to assets still held at the reporting date	(4.2)	5.4
Relating to assets sold during the period	28.6	0.0
Purchases	0.0	0.0
Sales	(23.8)	(0.4)
Fair value of plan assets — end of year	70.4	70.5
Significant Unobservable Inputs (Level 3) [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	36.0	35.2
Actual return on plan assets:
Relating to assets still held at the reporting date	1.4	1.6
Relating to assets sold during the period	0.1	0.3
Purchases	0.0	0.0
Sales	(3.9)	(1.1)
Fair value of plan assets — end of year	33.6	36.0
Significant Unobservable Inputs (Level 3) [Member] | Other Postretirement Benefits Plan [Member] | Hedge Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	11.4	11.4
Actual return on plan assets:
Relating to assets still held at the reporting date	1.0	0.0
Relating to assets sold during the period	0.0	0.0
Purchases	0.0	0.0
Sales	0.0	0.0
Fair value of plan assets — end of year	12.4	11.4
Significant Unobservable Inputs (Level 3) [Member] | Other Postretirement Benefits Plan [Member] | Private Equity Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	3.0	3.9
Actual return on plan assets:
Relating to assets still held at the reporting date	(0.6)	(0.1)
Relating to assets sold during the period	0.0	0.3
Purchases	0.0	0.0
Sales	(0.7)	(1.1)
Fair value of plan assets — end of year	1.7	3.0
Significant Unobservable Inputs (Level 3) [Member] | Other Postretirement Benefits Plan [Member] | Structured Credit [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	8.5	7.9
Actual return on plan assets:
Relating to assets still held at the reporting date	0.1	0.6
Relating to assets sold during the period	0.1	0.0
Purchases	0.0	0.0
Sales	(3.2)	0.0
Fair value of plan assets — end of year	5.5	8.5
Significant Unobservable Inputs (Level 3) [Member] | Other Postretirement Benefits Plan [Member] | Real Estate [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets — beginning of year	13.1	12.0
Actual return on plan assets:
Relating to assets still held at the reporting date	0.9	1.1
Relating to assets sold during the period	0.0	0.0
Purchases	0.0	0.0
Sales	0.0	0.0
Fair value of plan assets — end of year	$ 14.0	$ 13.1