Property, Plant and Equipment (details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Public Utilities Property Plant And Equipment Abstract
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service	2.80%	2.80%	2.90%
Weighted Average Depreciation Rates Non Utility	2.20%	2.50%
Asset Removal Costs	$ 267,400,000	$ 252,900,000
Property Plant And Equipment Net By Type Abstract
Property, plant and equipment	2,661,102,000	2,576,402,000
Less: Accumulated depreciation	767,226,000	722,239,000
Total property, plant and equipment - net	1,893,876,000	1,854,163,000
Utility Plant [Member]
Property Plant And Equipment Net By Type Abstract
Property, plant and equipment	2,323,467,000	2,247,952,000
Public Utilities Property Plant And Equipment Construction Work In Progress	36,051,000	29,324,000
Less: Accumulated depreciation	749,603,000	710,214,000
Total property, plant and equipment - net	1,609,915,000	1,567,062,000
Non Utility Plant [Member]
Property Plant And Equipment Net By Type Abstract
Property, plant and equipment	293,205,000	290,038,000
Public Utilities Property Plant And Equipment Construction Work In Progress	8,379,000	9,088,000
Less: Accumulated depreciation	17,623,000	12,025,000
Total property, plant and equipment - net	$ 283,961,000	$ 287,101,000