Components of deferred tax assets and liabilities (Details 3) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Valuation Allowance, Amount	$ 0	$ 0
Deferred tax assets:
Regulatory matters	125,607,000	121,781,000
Accrued pension costs	74,320,000	85,037,000
Alternative minimum tax credit carryforward	33,304,000	0
Compensation-related	31,550,000	23,441,000
Asset retirement obligations	29,578,000	26,748,000
Legal and environmental contingencies	10,710,000	8,046,000
Other	45,101,000	39,792,000
Total deferred tax assets	350,170,000	304,845,000
Deferred tax liabilities:
Depreciation and basis differences on property, plant and equipment	813,597,000	755,392,000
Basis differences on oil and natural gas producing properties	266,168,000	167,113,000
Regulatory matters	64,914,000	82,077,000
Intangible asset amortization	13,579,000	14,078,000
Other	26,170,000	18,441,000
Total deferred tax liabilities	1,184,428,000	1,037,101,000
Net deferred income tax liability	$ (834,258,000)	$ (732,256,000)