REGULATORY ASSETS AND LIABILITIES - Regulatory Liabilities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	$ 63.1	$ 49.0
Regulatory liabilities, noncurrent	53.7	66.6
RSE adjustment
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	19.8	4.7
Regulatory liabilities, noncurrent	0	0
Unbilled service margin
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	5.2	28.5
Regulatory liabilities, noncurrent	0	0
Postretirement liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	2.3	0
Regulatory liabilities, noncurrent	26.2	26.2
Refundable negative salvage
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	13.4	15.8
Regulatory liabilities, noncurrent	26.8	39.7
Gas supply adjustment
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	22.4	0
Regulatory liabilities, noncurrent	0	0
RSE adjustment
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	0	0
Regulatory liabilities, noncurrent	$ 0.7	$ 0.7