Income Taxes (Details 4) (USD $)	12 Months Ended
	Dec. 31, 2011 Y	Dec. 31, 2010 Y	Dec. 31, 2009	Dec. 31, 2008 Y
Deferred income taxes:
Total deferred income tax assets	$ (449,888,000)	$ (385,948,000)
Total deferred income tax liabilities	2,038,225,000	1,622,520,000
Total net deferred income tax liabilities	1,588,337,000	1,236,572,000
Total deferred income tax liabilities (assets)
Tax depreciation in excess of book depreciation	1,037,691,000	918,567,000
Drilling and development costs expensed for income tax reporting	836,219,000	632,985,000
Accumulated other comprehensive income (loss)	120,295,000	21,217,000
Regulatory temporary differences	43,005,000	44,335,000
Deferred purchased gas cost	1,015,000	5,113,000
Financial instruments	(1,303,000)	302,000
Incentive compensation	(1,324,000)	(15,971,000)
Investment tax credit	(1,467,000)	(1,856,000)
Uncollectible accounts	(3,953,000)	(4,092,000)
Post-retirement benefits	(8,140,000)	(6,630,000)
Deferred compensation plans	(10,814,000)	(15,698,000)
Alternative minimum tax credit carryforward	(65,509,000)	(26,017,000)
Net operating loss carryforwards	(337,921,000)	(305,787,000)
Other	(19,457,000)	(9,896,000)
Total (including amounts classified as current (assets) of ($26,867) and ($33,586), respectively)	1,588,337,000	1,236,572,000
Current deferred income tax assets	26,867,000	33,586,000
Deferred assets/(liability) related to net unrealized income/loss from hedging transactions	(149,800,000)	(39,600,000)
Deferred tax asset (liability) related to net unrealized (gain) loss on available-for-sale securities		(2,600,000)
Deferred tax asset related to other post-retirement benefits	7,800,000	7,000,000
Deferred tax asset related to pension plans	15,900,000	14,000,000
Deferred tax asset related to interest rate swaps	5,700,000
Total deferred tax asset related to the federal net operating loss carryforward	278,800,000
Deferred tax asset related to the net operating loss carryforward	54,900,000	229,200,000		2,200,000
Net operating loss carryforward period (in years)	20	20		20
AMT carryforwards generated	65,500,000
Deferred tax asset for state net operating loss carryforwards	59,100,000	52,200,000
Reduction in deferred tax asset for uncertain tax positions related to federal operating loss carryforwards	7,500,000	800,000
Reduction in deferred tax asset for uncertain tax positions related to state operating loss carryforwards	8,800,000	5,700,000
Valuation allowance pertaining to operating loss carry forward	800,000	3,300,000
Excess tax benefit	6,600,000	5,000,000
Income tax benefit by the exercise of nonqualified employee stock options and vesting of restricted share awards			$ 1,000,000