Commitments and Contingencies (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended		85 Months Ended	6 Months Ended	15 Months Ended		6 Months Ended			1 Months Ended		6 Months Ended
	Jun. 30, 2012 plaintiff MW	Jun. 30, 2012 Elk River Windfarm MW	Dec. 31, 2012 Elk River Windfarm Y	Jun. 30, 2012 Southern Star Central Pipeline, Inc. Electric	Jun. 30, 2012 Southern Star Central Pipeline, Inc. Electric dth	Jun. 30, 2012 Iatan	Jun. 30, 2012 Physical gas and transportation contracts	Jun. 30, 2012 Coal and coal transportation contracts	Jun. 30, 2012 Purchased power Plum Point Energy Station Y MW	Dec. 31, 2008 Purchased power Cloud County Windfarm Y	Dec. 31, 2005 Purchased power Cloud County Windfarm Y	Jun. 30, 2012 Purchased power Cloud County Windfarm MW	Jun. 30, 2012 Leases Electric train	Jun. 30, 2012 Leases Gas office	Jun. 30, 2012 Leases Plum Point Energy Station railcar	Jun. 30, 2012 Purchased power agreement in 2015 Plum Point Energy Station
Commitments and Contingencies
Amount of total power capacity of generating facility (in megawatts)	18
Proportionate ownership interest (as a percent)						12.00%
July 1, 2012 through December 31, 2012							$ 19.7	$ 20.2
January 1, 2013 through December 31, 2014							45.8	47.1
January 1, 2015 through December 31, 2016							24.5	32.2
January 1, 2017 and beyond							17.8
Term of long-term contract agreement (in years)			20						30
Agreement to purchase firm gas storage capacity (in Dths)					1,000,000
Term of long-term contract agreement					5 years
Amount of annual storage capacity reservation				1.1
Amount of long-term contract obligation									$ 311.3							$ 30.4
Amount of energy generated (in megawatts)		150							665			105
Amount of energy capacity under option to purchase (in megawatts)									50
Period of the average cost that is used as a basis for determining the maximum annual payments (in years)										20	20
Number of unit trains under short-term operating leases													2
Number of office facilities under short-term operating leases														1
Number of railcars under capital leases															108
Number of plaintiffs	3