Electric utility subsidiary (Details 3) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 Y	Dec. 31, 2010 Y
Regulatory assets
Regulatory assets	$ 669,389	$ 478,330
Regulatory liabilities
Regulatory liabilities	315,466	296,797
Electric utility
Regulatory assets
Regulatory assets	669,389	478,330
Regulatory liabilities
Regulatory liabilities	315,466	296,797
Electric utility | Cost of removal in excess of salvage value
Regulatory liabilities
Regulatory liabilities	294,817	277,341
Electric utility | Retirement benefit plans, regulatory liabilities
Regulatory liabilities
Authorized amortization or recovery periods (in years)	5	5
Remaining amortization or recovery periods (in years)	5	5
Regulatory liabilities	20,000	18,617
Electric utility | Other, regulatory liabilities
Regulatory liabilities
Authorized amortization or recovery periods (in years)	5	5
Regulatory liabilities	649	839
Electric utility | Retirement benefit plans, regulatory assets
Regulatory assets
Regulatory assets	523,640	356,591
Electric utility | Income taxes, net
Regulatory assets
Regulatory assets	83,386	82,615
Electric utility | Decoupling revenue balancing account
Regulatory assets
Authorized amortization or recovery periods (in years)	1
Regulatory assets	20,780
Electric utility | Unamortized expense and premiums on retired debt and equity issuances
Regulatory assets
Regulatory assets	12,267	13,589
Electric utility | Vacation earned, but not yet taken
Regulatory assets
Authorized amortization or recovery periods (in years)	1	1
Regulatory assets	8,161	7,349
Electric utility | Postretirement benefits other than pensions
Regulatory assets
Authorized amortization or recovery periods (in years)	18	18
Remaining amortization or recovery periods (in years)	1	1
Regulatory assets	1,861	3,579
Electric utility | Other, regulatory assets
Regulatory assets
Regulatory assets	$ 19,294	$ 14,607
Electric utility | Maximum | Cost of removal in excess of salvage value
Regulatory liabilities
Authorized amortization or recovery periods (in years)	60	60
Electric utility | Maximum | Other, regulatory liabilities
Regulatory liabilities
Remaining amortization or recovery periods (in years)	5	5
Electric utility | Maximum | Income taxes, net
Regulatory assets
Authorized amortization or recovery periods (in years)	48	48
Electric utility | Maximum | Unamortized expense and premiums on retired debt and equity issuances
Regulatory assets
Authorized amortization or recovery periods (in years)	30	30
Remaining amortization or recovery periods (in years)	17	17
Electric utility | Maximum | Other, regulatory assets
Regulatory assets
Authorized amortization or recovery periods (in years)	50	50
Remaining amortization or recovery periods (in years)	48	48
Electric utility | Minimum | Cost of removal in excess of salvage value
Regulatory liabilities
Authorized amortization or recovery periods (in years)	1	1
Electric utility | Minimum | Other, regulatory liabilities
Regulatory liabilities
Remaining amortization or recovery periods (in years)	1	1
Electric utility | Minimum | Income taxes, net
Regulatory assets
Authorized amortization or recovery periods (in years)	1	1
Electric utility | Minimum | Unamortized expense and premiums on retired debt and equity issuances
Regulatory assets
Authorized amortization or recovery periods (in years)	14	14
Remaining amortization or recovery periods (in years)	1	1
Electric utility | Minimum | Other, regulatory assets
Regulatory assets
Authorized amortization or recovery periods (in years)	1	1
Remaining amortization or recovery periods (in years)	1	1