Summary of Significant Accounting Policies (Details 3) (USD $)	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Property, Plant and Equipment [Line Items]
Allowance for Funds Used During Construction, Capitalized Cost of Equity and Interest	$ 15,100,000		$ 17,600,000		$ 17,400,000
Property, plant and equipment	7,646,222,000		7,218,503,000
Less accumulated depreciation, depletion and amortization	3,361,208,000		3,103,323,000		2,872,465,000
Net property, plant and equipment	4,285,014,000		4,115,180,000		3,894,117,000
Mcf equivalent average rate for amortization under units-of-production method (in dollars per Mcf)	2.04		1.77		1.64
Capitalized Costs of Unproved Properties Excluded from Amortization, Cumulative	232,462,000		182,402,000		178,214,000
Electric [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	1,068,524,000	[1]	1,027,034,000	[1]	941,791,000	[1]
Natural gas distribution [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	1,568,866,000	[1]	1,508,845,000	[1]	1,456,208,000	[1]
Pipeline and energy services [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	719,291,000		683,807,000		675,199,000
Exploration and production [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	2,615,146,000		2,356,938,000		2,028,794,000
Construction materials and contracting [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	1,499,852,000		1,486,375,000		1,514,989,000
Construction services [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	124,796,000		122,940,000		116,236,000
All Other Segments [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	49,747,000		32,564,000		33,365,000
Regulated Operation [Member] | Electric [Member] | Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	546,783,000		538,071,000
Weighted Average Depreciable Life in Years	47
Regulated Operation [Member] | Electric [Member] | Electric Distribution [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	255,232,000		243,205,000
Weighted Average Depreciable Life in Years	36
Regulated Operation [Member] | Electric [Member] | Electric Transmission [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	179,580,000		161,972,000
Weighted Average Depreciable Life in Years	44
Regulated Operation [Member] | Electric [Member] | Other Capitalized Property Plant and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	86,929,000		83,786,000
Weighted Average Depreciable Life in Years	13
Regulated Operation [Member] | Natural gas distribution [Member] | Distribution [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	1,257,360,000		1,223,239,000
Weighted Average Depreciable Life in Years	38
Regulated Operation [Member] | Natural gas distribution [Member] | Other Capitalized Property Plant and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	311,506,000		285,606,000
Weighted Average Depreciable Life in Years	23
Regulated Operation [Member] | Pipeline and energy services [Member] | Transmission [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	386,227,000		357,395,000
Weighted Average Depreciable Life in Years	52
Regulated Operation [Member] | Pipeline and energy services [Member] | Gathering [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	42,378,000		41,931,000
Weighted Average Depreciable Life in Years	19
Regulated Operation [Member] | Pipeline and energy services [Member] | Storage [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	41,908,000		33,967,000
Weighted Average Depreciable Life in Years	51
Regulated Operation [Member] | Pipeline and energy services [Member] | Other Capitalized Property Plant and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	36,179,000		33,938,000
Weighted Average Depreciable Life in Years	29
Unregulated Operation [Member] | Pipeline and energy services [Member] | Gathering [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	198,864,000		203,064,000
Weighted Average Depreciable Life in Years	17
Unregulated Operation [Member] | Pipeline and energy services [Member] | Other Capitalized Property Plant and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	13,735,000		13,512,000
Weighted Average Depreciable Life in Years	10
Unregulated Operation [Member] | Exploration and production [Member] | Oil and natural gas properties [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	2,577,576,000	[2]	2,320,967,000	[2]
Unregulated Operation [Member] | Exploration and production [Member] | Other Capitalized Property Plant and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	37,570,000		35,971,000
Weighted Average Depreciable Life in Years	9
Unregulated Operation [Member] | Construction materials and contracting [Member] | Land [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	126,790,000		124,018,000
Unregulated Operation [Member] | Construction materials and contracting [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	67,627,000		65,003,000
Weighted Average Depreciable Life in Years	20
Unregulated Operation [Member] | Construction materials and contracting [Member] | Machinery, vehicles and equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	902,136,000		899,365,000
Weighted Average Depreciable Life in Years	12
Unregulated Operation [Member] | Construction materials and contracting [Member] | Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	8,085,000		4,879,000
Unregulated Operation [Member] | Construction materials and contracting [Member] | Aggregate reserves [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	395,214,000	[3]	393,110,000	[3]
Unregulated Operation [Member] | Construction services [Member] | Land [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	4,706,000		4,526,000
Unregulated Operation [Member] | Construction services [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	15,001,000		14,101,000
Weighted Average Depreciable Life in Years	22
Unregulated Operation [Member] | Construction services [Member] | Machinery, vehicles and equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	95,891,000		94,252,000
Weighted Average Depreciable Life in Years	7
Unregulated Operation [Member] | Construction services [Member] | Other Capitalized Property Plant and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	9,198,000		10,061,000
Weighted Average Depreciable Life in Years	4
Unregulated Operation [Member] | All Other Segments [Member] | Land [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	2,837,000		2,837,000
Unregulated Operation [Member] | All Other Segments [Member] | Other Capitalized Property Plant and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	$ 46,910,000		$ 29,727,000
Weighted Average Depreciable Life in Years	24

[1] Includes allocations of common utility property.

[2] Amortized on the units-of-production method based on total proved reserves at an Mcf equivalent average rate of $2.04, $1.77 and $1.64 for the years ended December 31, 2011, 2010 and 2009, respectively. Includes natural gas and oil properties accounted for under the full-cost method, of which $232.5 and $182.4 million were excluded from amortization at December 31, 2011 and 2010, respectively.

[3] Depleted on the units-of-production method.