EMPLOYEE BENEFITS - Schedule of Asset Category (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 4,968.6	$ 4,573.9	$ 4,739.5
Uncategorized	4,159.1	4,046.4
Pension Plan | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	1,942.5	1,821.2
Uncategorized	1,349.9	1,377.8
Pension Plan | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	1,625.2	1,511.8
Uncategorized	1,222.8	1,265.5
Pension Plan | Private Equity
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	988.3	840.1
Uncategorized	971.4	834.0
Pension Plan | Real Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	673.7	632.0
Uncategorized	615.0	569.1
Pension Plan | Total Assets Before 401
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	5,229.7	4,805.1
Pension Plan | 401 PBOP Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	(261.1)	(231.2)
Pension Plan | Fair Value, Inputs, Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	767.6	597.9
Pension Plan | Fair Value, Inputs, Level 1 | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	592.6	443.4
Pension Plan | Fair Value, Inputs, Level 1 | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	99.4	85.5
Pension Plan | Fair Value, Inputs, Level 1 | Private Equity
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	16.9	6.1
Pension Plan | Fair Value, Inputs, Level 1 | Real Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	58.7	62.9
Pension Plan | Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	303.0	160.8
Pension Plan | Level 2 | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Pension Plan | Level 2 | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	303.0	160.8
Pension Plan | Level 2 | Private Equity
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Pension Plan | 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	935.9	849.6	$ 922.2
Uncategorized	410.2	436.7
Other Postretirement Benefits Plan | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	345.0	302.4
Uncategorized	187.0	210.5
Other Postretirement Benefits Plan | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	203.5	185.3
Uncategorized	148.1	123.0
Other Postretirement Benefits Plan | Private Equity
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	26.5	32.7
Uncategorized	26.5	32.7
Other Postretirement Benefits Plan | Real Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	99.8	98.0
Uncategorized	48.6	70.5
Other Postretirement Benefits Plan | Total Assets Before 401
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	674.8	618.4
Other Postretirement Benefits Plan | 401 PBOP Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	261.1	231.2
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	225.0	141.4
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1 | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	158.0	91.9
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1 | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	15.8	22.0
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	51.2	27.5
Other Postretirement Benefits Plan | Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	39.6	40.3
Other Postretirement Benefits Plan | Level 2 | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Other Postretirement Benefits Plan | Level 2 | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	39.6	40.3
Other Postretirement Benefits Plan | Level 2 | Private Equity
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Other Postretirement Benefits Plan | Level 2 | Real Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 0.0	$ 0.0