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ACQUISITION OF FOX ENERGY CENTER (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended			0 Months Ended	1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 02, 2014	Mar. 31, 2013 MW
Acquisitions
Purchase price	$ 0	$ 391.6	$ 0
Fox Energy Company LLC
Acquisitions
Purchase price					391.6
Capacity of Electric Generating Facility					593
Assets acquired
Inventories - materials and supplies					3.0
Other current assets					0.4
Property, plant, and equipment					374.4
Other long-term assets					15.6
Total assets acquired					393.4
Liabilities assumed
Accounts payable					1.8
Total liabilities assumed					1.8
Contracted Capacity From Power Purchase Agreement					500
Contract termination fee related to tolling arrangement					$ 50.0
Amortization Period of Regulatory Asset	9 years
Beginning of regulatory asset amortization period				Jan. 01, 2014