EMPLOYEE BENEFITS - Schedule of Asset category (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 4,739.5	$ 4,076.0	$ 3,905.4
Uncategorized	4,053.3	3,558.4
Pension Plan | Equity Securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	2,188.7	1,735.2
Uncategorized	1,653.3	1,279.7
Pension Plan | Private Equity
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	653.0	524.4
Uncategorized	641.8	518.4
Pension Plan | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	1,490.8	1,282.4
Uncategorized	1,218.3	1,099.4
Pension Plan | Real Estate and Other Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	476.0	403.1
Uncategorized	374.4	325.9
Pension Plan | Hedge Funds
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	165.5	335.0
Uncategorized	165.5	335.0
Pension Plan | Total Assets Before 401 (h)
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	4,974.0	4,280.1
Pension Plan | 401(h) PBOP Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	234.5	204.1
Pension Plan | Fair Value, Inputs, Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	704.8	538.7
Pension Plan | Fair Value, Inputs, Level 1 | Equity Securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	535.4	455.5
Pension Plan | Fair Value, Inputs, Level 1 | Private Equity
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	11.2	6.0
Pension Plan | Fair Value, Inputs, Level 1 | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	56.6	0.0
Pension Plan | Fair Value, Inputs, Level 1 | Real Estate and Other Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	101.6	77.2
Pension Plan | Fair Value, Inputs, Level 1 | Hedge Funds
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Pension Plan | Fair Value, Inputs, Level 2
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	215.9	183.0
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 | 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 | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	215.9	183.0
Pension Plan | Fair Value, Inputs, Level 2 | Real Estate and Other Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Pension Plan | Fair Value, Inputs, Level 2 | Hedge Funds
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	922.2	815.8	$ 812.2
Uncategorized	482.6	453.3
Other Postretirement Benefits Plan | Equity Securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	357.2	302.7
Uncategorized	241.9	214.1
Other Postretirement Benefits Plan | Private Equity
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	31.3	32.2
Uncategorized	31.3	32.2
Other Postretirement Benefits Plan | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	201.3	186.6
Uncategorized	133.9	132.3
Other Postretirement Benefits Plan | Real Estate and Other Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	51.4	43.0
Uncategorized	29.0	27.5
Other Postretirement Benefits Plan | Hedge Funds
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	46.5	47.2
Uncategorized	46.5	47.2
Other Postretirement Benefits Plan | Total Assets Before 401 (h)
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	687.7	611.7
Other Postretirement Benefits Plan | 401(h) PBOP Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	234.5	204.1
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	161.1	113.6
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1 | Equity Securities
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	115.3	88.6
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 | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	23.4	9.5
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1 | Real Estate and Other Assets
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	22.4	15.5
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1 | Hedge Funds
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
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	44.0	44.8
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 | 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 | Fixed Income
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	44.0	44.8
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 2 | Real Estate and Other Assets
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 | Hedge Funds
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 0.0	$ 0.0