EMPLOYEE BENEFITS - Schedule of Asset Category (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 6,495.5	$ 5,409.2	$ 4,968.6
Uncategorized	5,479.8	4,480.4
Pension Plan | Total Assets Before 401
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	6,794.0	5,671.1
Pension Plan | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	2,107.7	1,952.5
Uncategorized	1,385.2	1,321.7
Pension Plan | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	2,062.5	1,781.7
Uncategorized	1,689.1	1,402.5
Pension Plan | Private Equity
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	1,702.7	1,197.7
Uncategorized	1,702.7	1,175.4
Pension Plan | Real Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	921.1	739.2
Uncategorized	702.8	580.8
Pension Plan | 401 PBOP Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	(298.5)	(261.9)
Pension Plan | Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	1,080.4	925.1
Pension Plan | Level 1 | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	722.5	630.8
Pension Plan | Level 1 | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	139.6	113.6
Pension Plan | Level 1 | Private Equity
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	22.3
Pension Plan | Level 1 | Real Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	218.3	158.4
Pension Plan | Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	233.8	265.6
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	233.8	265.6
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	1,138.3	1,004.1	$ 935.9
Uncategorized	463.5	424.4
Other Postretirement Benefits Plan | Total Assets Before 401
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	839.8	742.2
Other Postretirement Benefits Plan | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	439.7	394.3
Uncategorized	248.3	217.8
Other Postretirement Benefits Plan | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	220.4	212.1
Uncategorized	125.5	152.9
Other Postretirement Benefits Plan | Private Equity
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	58.7	31.5
Uncategorized	58.7	31.5
Other Postretirement Benefits Plan | Real Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	121.0	104.3
Uncategorized	31.0	22.2
Other Postretirement Benefits Plan | 401 PBOP Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	298.5	261.9
Other Postretirement Benefits Plan | Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	331.1	274.6
Other Postretirement Benefits Plan | Level 1 | Equity securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	191.4	176.5
Other Postretirement Benefits Plan | Level 1 | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	49.7	16.0
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	90.0	82.1
Other Postretirement Benefits Plan | Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	45.2	43.2
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	45.2	43.2
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