REGULATORY ASSETS AND LIABILITIES - Regulatory Assets (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Regulatory Assets [Line Items]
Regulatory assets, current	$ 8.8	$ 2.8
Regulatory assets, noncurrent	90.6	84.9
Postretirement assets
Regulatory Assets [Line Items]
Regulatory assets, current	6.4	0.3
Regulatory assets, noncurrent	66.1	58.2
Accretion and depreciation for asset retirement obligation
Regulatory Assets [Line Items]
Regulatory assets, current	0	0
Regulatory assets, noncurrent	18.3	18.0
Rate recovery of asset removal costs, net
Regulatory Assets [Line Items]
Regulatory assets, current	0	0
Regulatory assets, noncurrent	2.8	4.7
Enhanced stability reserve
Regulatory Assets [Line Items]
Regulatory assets, current	0	0
Regulatory assets, noncurrent	3.4	4.0
Gas supply adjustment
Regulatory Assets [Line Items]
Regulatory assets, current	0	2.5
Regulatory assets, noncurrent	0	0
RSE adjustment
Regulatory Assets [Line Items]
Regulatory assets, current	2.4	0
Regulatory assets, noncurrent	$ 0	$ 0