Electric utility subsidiary (Details 2) (USD $) In Thousands, except Share data, unless otherwise specified										12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2011 Cumulative Preferred Stock $20 Par Value	Dec. 31, 2011 Cumulative Preferred Stock $100 Par Value	Dec. 31, 2011 Electric utility	Dec. 31, 2010 Electric utility	Dec. 31, 2009 Electric utility	Dec. 31, 2011 Electric utility Minimum	Dec. 31, 2011 Electric utility Maximum
Utility plant, at cost
Property, plant and equipment							$ 5,103,541	$ 4,948,338
Less accumulated depreciation							(1,966,894)	(1,941,059)
Construction in progress							138,838	101,562
Net utility plant							3,275,485	3,108,841
Regulatory assets	669,389	478,330					669,389	478,330
Other							727,068	698,509
Total assets	9,592,731	9,085,344					4,671,942	4,285,680
Capitalization and liabilities
Common stock ($6 2/3 par value, authorized 50,000,000 shares, outstanding 13,830,823 shares and 13,786,959 shares in 2010 and 2009, respectively)							94,911	92,224
Premium on capital stock							426,921	389,609
Retained earnings	201,640	181,910					884,284	854,856
Accumulated other comprehensive income (loss), net of income taxes	(19,137)	(12,472)					(32)	709
Total shareholders' equity	1,531,949	1,483,637	1,441,648	1,389,454			1,406,084	1,337,398
Cumulative preferred stock - not subject to mandatory redemption							34,293	34,293
Long-term debt, net		1,364,942					1,000,570	1,057,942
Total capitalization							2,440,947	2,429,633
Current portion of long-term debt							57,500
Deferred income taxes	354,051	278,958					337,863	269,286
Regulatory liabilities	315,466	296,797					315,466	296,797
Contributions in aid of construction	356,203	335,364					356,203	335,364
Other	516,990	446,485					1,163,963	954,600
Total liabilities and shareholders' equity	$ 9,592,731	$ 9,085,344					$ 4,671,942	$ 4,285,680
Common stock
Common stock, par value (in dollars per share)							$ 6.667	$ 6.667	$ 6.667
Common stock, authorized shares	200,000,000	200,000,000					50,000,000	50,000,000	50,000,000
Common stock, outstanding shares	96,038,328	94,690,932					14,233,723	13,830,823	13,786,959
Preferred stock
Cumulative preferred stock, par value (in dollars per share)					$ 20	$ 100	$ 100	$ 100
Preferred stock, authorized shares	10,000,000	10,000,000			5,000,000	7,000,000	7,000,000	7,000,000
Cumulative preferred stock, outstanding shares					1,114,657	120,000	120,000	120,000
Preferred stock, Dividend Rate (as a percent)										4.25%	7.625%