Income Taxes (Details 3) (USD $)	Dec. 31, 2011	Dec. 31, 2010
Valuation Allowance, Amount	$ 0	$ 0
Deferred tax assets [Abstract]
Regulatory matters	119,189,000	114,427,000
Accrued pension costs	95,260,000	82,085,000
Asset retirement obligations	26,380,000	24,391,000
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Legal Settlements	21,788,000	13,622,000
Compensation-related	16,241,000	17,261,000
Other	41,055,000	40,307,000
Total deferred tax assets	319,913,000	292,093,000
Deferred tax liabilities [Abstract]
Depreciation and basis differences on property, plant and equipment	715,482,000	679,809,000
Basis differences on natural gas and oil producing properties	210,146,000	152,455,000
Regulatory matters	84,963,000	64,017,000
Intangible asset amortization	14,307,000	14,843,000
Other	23,774,000	20,348,000
Total deferred tax liabilities	1,048,672,000	931,472,000
Net deferred income tax liability	$ (728,759,000)	$ (639,379,000)