EMPLOYEE BENEFITS - Schedule of Asset category (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 4,573.9	$ 4,739.5	$ 4,076.0
Uncategorized	4,046.4	4,053.3
Pension Plan | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	1,821.2	2,188.7
Uncategorized	1,377.8	1,653.3
Pension Plan | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	1,511.8	1,490.8
Uncategorized	1,265.5	1,218.3
Pension Plan | Private Equity
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	840.1	653.0
Uncategorized	834.0	641.8
Pension Plan | Real Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	632.0	641.5
Uncategorized	569.1	539.9
Pension Plan | Total Assets Before 401
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	4,805.1	4,974.0
Pension Plan | 401 PBOP Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	(231.2)	(234.5)
Pension Plan | Fair Value, Inputs, Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	597.9	704.8
Pension Plan | Fair Value, Inputs, Level 1 | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	443.4	535.4
Pension Plan | Fair Value, Inputs, Level 1 | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	85.5	56.6
Pension Plan | Fair Value, Inputs, Level 1 | Private Equity
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	6.1	11.2
Pension Plan | Fair Value, Inputs, Level 1 | Real Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	62.9	101.6
Pension Plan | Fair Value, Inputs, Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	160.8	215.9
Pension Plan | Fair Value, Inputs, Level 2 | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Pension Plan | Fair Value, Inputs, Level 2 | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	160.8	215.9
Pension Plan | Fair Value, Inputs, Level 2 | Private Equity
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Pension Plan | Fair Value, Inputs, Level 2 | Real Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	849.6	922.2	$ 815.8
Uncategorized	436.7	482.6
Other Postretirement Benefits Plan | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	302.4	357.2
Uncategorized	210.5	241.9
Other Postretirement Benefits Plan | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	185.3	201.3
Uncategorized	123.0	133.9
Other Postretirement Benefits Plan | Private Equity
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	32.7	31.3
Uncategorized	32.7	31.3
Other Postretirement Benefits Plan | Real Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	98.0	97.9
Uncategorized	70.5	75.5
Other Postretirement Benefits Plan | Total Assets Before 401
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	618.4	687.7
Other Postretirement Benefits Plan | 401 PBOP Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	231.2	234.5
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	141.4	161.1
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1 | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	91.9	115.3
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1 | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	22.0	23.4
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1 | Private Equity
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1 | Real Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	27.5	22.4
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	40.3	44.0
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 2 | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 2 | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	40.3	44.0
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 2 | Private Equity
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 2 | Real Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 0.0	$ 0.0