Regulatory Matters (Details) (USD $)	9 Months Ended
	Sep. 30, 2011 segment M Y town	Dec. 31, 2010
Regulatory Assets and Liabilities
Amortization period (in years)	eighteen year
Regulatory Assets:
Regulatory assets, current	$ 8,770,000	$ 4,974,000
Regulatory assets, long-term	191,484,000	189,404,000
Regulatory assets, total	200,254,000	194,378,000
Regulatory Liabilities:
Regulatory liabilities, current	120,000	1,243,000
Regulatory liabilities, long-term	124,444,000	87,579,000
Regulatory liabilities, total	124,564,000	88,822,000
Period to recover costs in Missouri rates (in months)	12
Iatan2
Regulatory Assets and Liabilities
Deferred fuel and purchased power expense	8,300,000
Deferral recovery period, minimum (in years)	3
Deferral recovery period, maximum (in years)	5
Plum point
Regulatory Assets and Liabilities
Deferral recovery period, minimum (in years)	3
Deferral recovery period, maximum (in years)	5
Electric | Unsettled derivative losses
Regulatory Assets:
Regulatory assets, long-term	4,505,000	3,166,000
Electric | Over recovered electric fuel and purchased power costs
Regulatory Liabilities:
Regulatory liabilities, long-term	175,000	155,000
Gas | Under recovered purchased gas costs
Regulatory Assets:
Regulatory assets, long-term	1,159,000	439,000
Gas | Over recovered purchased gas costs
Regulatory Liabilities:
Regulatory liabilities, current	120,000	1,243,000
Missouri jurisdictional portion of the payment, net of tax
Regulatory Assets and Liabilities
Amortization period (in years)	ten years
Arkansas jurisdictional portion of the 2010 SWPA payment
Regulatory Assets and Liabilities
Amortization period (in years)	50 year
Under recovered electric fuel and purchased power costs
Regulatory Assets:
Regulatory assets, current	8,770,000	4,974,000
Pension and other postretirement benefits
Regulatory Assets:
Regulatory assets, long-term	85,700,000	92,192,000
Regulatory Liabilities:
Pension and other postretirement benefit costs recognized	400,000
Pension and other postretirement benefit costs reduction recognized	500,000
Pension and other postretirement benefits
Regulatory Liabilities:
Regulatory liabilities, long-term	3,210,000	4,604,000
Income taxes
Regulatory Assets:
Regulatory assets, long-term	49,707,000	50,188,000
Income taxes
Regulatory Liabilities:
Regulatory liabilities, long-term	12,344,000	12,715,000
Unamortized loss on reacquired debt
Regulatory Assets:
Regulatory assets, long-term	11,982,000	13,099,000
Deferred operating and maintenance expenses
Regulatory Assets:
Regulatory assets, long-term	442,000
Unamortized loss on interest rate derivative
Regulatory Assets:
Regulatory assets, long-term	1,541,000	1,776,000
Asbury five-year maintenance
Regulatory Assets:
Regulatory assets, long-term	606,000	948,000
Storm costs
Regulatory Assets:
Regulatory assets, long-term	5,440,000	7,733,000
Deferred construction accounting costs.
Regulatory Assets:
Regulatory assets, long-term	17,012,000	10,521,000
Asset retirement obligation
Regulatory Assets:
Regulatory assets, long-term	3,532,000	3,412,000
Under recovered electric fuel and purchased gas costs
Regulatory Assets:
Regulatory assets, long-term	1,069,000
Customer programs
Regulatory Assets:
Regulatory assets, long-term	3,205,000	2,119,000
System reliability - vegetation management
Regulatory Assets:
Regulatory assets, long-term	4,871,000	3,338,000
Other
Regulatory Assets:
Regulatory assets, long-term	713,000	473,000
Regulatory Liabilities:
Regulatory liabilities, long-term		342,000
Cost of removal
Regulatory Liabilities:
Regulatory liabilities, long-term	70,932,000	62,756,000
SWPA payment for Ozark Beach lost generation
Regulatory Liabilities:
Regulatory liabilities, long-term	25,689,000
Unamortized gain on interest rate derivative
Regulatory Liabilities:
Regulatory liabilities, long-term	3,754,000	3,881,000
Deferred construction accounting costs - fuel
Regulatory Liabilities:
Regulatory liabilities, long-term	$ 8,340,000	$ 3,126,000