INCOME TAXES (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income tax expense:
Current state	$ 1.8	$ 5.6	$ 5.5
Deferred state	(0.9)	(0.7)	4.1
Total income tax expense	0.9	4.9	9.6
A reconciliation of the income tax expense with amounts determined by applying the statutory U.S. federal income tax rate to income before income tax expense:
Income (loss) before income tax expense	(40.4)	459.2	749.6
State income tax expense	$ 0.9	$ 4.9	$ 9.6
Effective income tax rate	(2.20%)	1.10%	1.30%
Current income tax payable	$ 1.0	$ 1.1
Deferred income tax liability	20.0	20.9
Accounting for Uncertainty in Income Taxes:
Unrecognized tax benefits at January 1	36.5	33.2	$ 29.8
Additions for tax positions taken in current period	2.3	3.3	3.4
Unrecognized tax benefits at December 31	$ 38.8	36.5	$ 33.2
State franchise tax rate (in percentage)	0.75%
Penalties accrued	$ 1.5	1.2
Interest accrued	1.3	0.8
Unrecognized tax benefits, possible estimate change	(2.8)
Accrued interest income not included in balance of unrecognized tax benefits	$ 1.0	$ 0.8
Tax Year 2013 [Member]
Accounting for Uncertainty in Income Taxes:
Tax year open to examination	2013
Tax Year 2014 [Member]
Accounting for Uncertainty in Income Taxes:
Tax year open to examination	2014
Tax Year 2015 [Member]
Accounting for Uncertainty in Income Taxes:
Tax year open to examination	2015
Texas State Income Tax Rate [Member]
A reconciliation of the income tax expense with amounts determined by applying the statutory U.S. federal income tax rate to income before income tax expense:
Effective income tax rate	0.40%	0.40%	0.40%
Deferred income tax liability	$ 5.4
Accounting for Uncertainty in Income Taxes:
State franchise tax rate (in percentage)	1.00%