EMPLOYEE BENEFITS - Schedule of Asset category (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 4,076.0	$ 3,905.4	$ 4,126.5
Uncategorized	3,558.4	3,334.0
Pension Plan [Member] | Equity Securities [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	1,735.2	1,687.4
Uncategorized	1,279.7	1,228.7
Pension Plan [Member] | Private Equity [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	524.4	472.3
Uncategorized	518.4	464.7
Pension Plan [Member] | Fixed Income [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	1,282.4	1,216.8
Uncategorized	1,099.4	1,008.2
Pension Plan [Member] | Real Estate and Other Assets [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	403.1	377.8
Uncategorized	325.9	291.9
Pension Plan [Member] | Hedge Funds [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	335.0	340.5
Uncategorized	335.0	340.5
Pension Plan [Member] | Total Assets Before 401 (h) [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	4,280.1	4,094.8
Pension Plan [Member] | 401(h) PBOP Assets [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	204.1	189.4
Pension Plan [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	538.7	404.1
Pension Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	455.5	396.5
Pension Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Private Equity [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	6.0	7.6
Pension Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Fixed Income [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Pension Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Real Estate and Other Assets [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	77.2	0.0
Pension Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Hedge Funds [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Pension Plan [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	183.0	356.7
Pension Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	62.2
Pension Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Private Equity [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Pension Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Fixed Income [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	183.0	208.6
Pension Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Real Estate and Other Assets [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	85.9
Pension Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Hedge Funds [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Other Postretirement Benefit Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	815.8	812.2	$ 862.6
Uncategorized	453.3	446.3
Other Postretirement Benefit Plan [Member] | Equity Securities [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	302.7	309.1
Uncategorized	214.1	199.4
Other Postretirement Benefit Plan [Member] | Private Equity [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	32.2	32.9
Uncategorized	32.2	32.9
Other Postretirement Benefit Plan [Member] | Fixed Income [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	186.6	191.2
Uncategorized	132.3	131.0
Other Postretirement Benefit Plan [Member] | Real Estate and Other Assets [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	43.0	37.4
Uncategorized	27.5	30.8
Other Postretirement Benefit Plan [Member] | Hedge Funds [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	47.2	52.2
Uncategorized	47.2	52.2
Other Postretirement Benefit Plan [Member] | Total Assets Before 401 (h) [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	611.7	622.8
Other Postretirement Benefit Plan [Member] | 401(h) PBOP Assets [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	204.1	189.4
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	113.6	119.4
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	88.6	109.7
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Private Equity [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Fixed Income [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	9.5	9.7
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Real Estate and Other Assets [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	15.5	0.0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Hedge Funds [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	44.8	57.1
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Private Equity [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	0.0
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Fixed Income [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	44.8	50.5
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Real Estate and Other Assets [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	0.0	6.6
Other Postretirement Benefit Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Hedge Funds [Member]
Defined Contribution Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 0.0	$ 0.0