Components of deferred tax assets and liabilities (Details 3) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Valuation Allowance, Amount	$ 0	$ 0
Deferred tax assets:
Regulatory matters	121,781,000	119,189,000
Accrued pension costs	85,037,000	95,260,000
Asset retirement obligations	26,748,000	26,380,000
Compensation-related	23,441,000	16,241,000
Legal and environmental contingencies	8,046,000	21,788,000
Other	39,792,000	41,055,000
Total deferred tax assets	304,845,000	319,913,000
Deferred tax liabilities:
Depreciation and basis differences on property, plant and equipment	755,392,000	715,482,000
Basis differences on oil and natural gas producing properties	167,113,000	210,146,000
Regulatory matters	82,077,000	84,963,000
Intangible asset amortization	14,078,000	14,307,000
Other	18,441,000	23,774,000
Total deferred tax liabilities	1,037,101,000	1,048,672,000
Net deferred income tax liability	$ (732,256,000)	$ (728,759,000)