EMPLOYEE BENEFITS - Schedule of Asset Category (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 5,409.2	$ 4,968.6	$ 4,573.9
Uncategorized	4,480.4	4,159.1
Pension Plan | Total Assets Before 401
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	5,671.1	5,229.7
Pension Plan | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	1,952.5	1,942.5
Uncategorized	1,321.7	1,349.9
Pension Plan | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	1,781.7	1,625.2
Uncategorized	1,402.5	1,222.8
Pension Plan | Private Equity
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	1,197.7	988.3
Uncategorized	1,175.4	971.4
Pension Plan | Real Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	739.2	673.7
Uncategorized	580.8	615.0
Pension Plan | 401 PBOP Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	(261.9)	(261.1)
Pension Plan | Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	925.1	767.6
Pension Plan | Level 1 | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	630.8	592.6
Pension Plan | Level 1 | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	113.6	99.4
Pension Plan | Level 1 | Private Equity
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	22.3	16.9
Pension Plan | Level 1 | Real Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	158.4	58.7
Pension Plan | Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	265.6	303.0
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	265.6	303.0
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	1,004.1	935.9	$ 849.6
Uncategorized	424.4	410.2
Other Postretirement Benefits Plan | Total Assets Before 401
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	742.2	674.8
Other Postretirement Benefits Plan | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	394.3	345.0
Uncategorized	217.8	187.0
Other Postretirement Benefits Plan | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	212.1	203.5
Uncategorized	152.9	148.1
Other Postretirement Benefits Plan | Private Equity
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	31.5	26.5
Uncategorized	31.5	26.5
Other Postretirement Benefits Plan | Real Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	104.3	99.8
Uncategorized	22.2	48.6
Other Postretirement Benefits Plan | 401 PBOP Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	261.9	261.1
Other Postretirement Benefits Plan | Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	274.6	225.0
Other Postretirement Benefits Plan | Level 1 | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	176.5	158.0
Other Postretirement Benefits Plan | Level 1 | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	16.0	15.8
Other Postretirement Benefits Plan | Level 1 | Private Equity
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Other Postretirement Benefits Plan | Level 1 | Real Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	82.1	51.2
Other Postretirement Benefits Plan | Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	43.2	39.6
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	43.2	39.6
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