Property, Plant and Equipment (Details) (USD $)			12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013 Land and improvements [Member] Minimum [Member]	Dec. 31, 2013 Land and improvements [Member] Maximum [Member]	Dec. 31, 2013 Building and improvements [Member] Minimum [Member]	Dec. 31, 2013 Building and improvements [Member] Maximum [Member]	Dec. 31, 2013 Tranmission, distribution and storage[Member] Minimum [Member]	Dec. 31, 2013 Tranmission, distribution and storage[Member] Maximum [Member]	Dec. 31, 2013 Generation [Member] Minimum [Member]	Dec. 31, 2013 Generation [Member] Maximum [Member]	Dec. 31, 2013 Plant Acquisition adjustment [Member]	Dec. 31, 2013 Other [Member] Minimum [Member]	Dec. 31, 2013 Other [Member] Maximum [Member]	Dec. 31, 2013 Basin Capital Lease [Member]	Dec. 31, 2012 Basin Capital Lease [Member]
Property, Plant and Equipment [Line Items]
Land, land rights and easements	$ 128,123,000	$ 72,550,000
Buildings and improvements	163,852,000	145,989,000
Transmission, distribution and storage	2,448,821,000	2,339,111,000
Generation	533,450,000	506,017,000
Plant acquisition adjustment	204,754,000	204,754,000
Other	308,345,000	259,308,000
Construction work in progress	104,891,000	121,360,000
Property, plant and equipment, gross	3,892,236,000	3,649,089,000
Less accumulated depreciation	(1,202,108,000)	(1,213,499,000)
Property, plant, and equipment, net	2,690,128,000	2,435,590,000
Estimated Useful Life			54 years	96 years	27 years	64 years	15 years	85 years	25 years	50 years	34 years	2 years	45 years
Property, plant, and equipment under capiltal leases	$ 25,600,000	$ 27,700,000												$ 25,100,000	$ 27,100,000