Income resulting from oil and natural gas producing activites (Details 4) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Revenues:
Sales to affiliates	$ 35,966,000		$ 93,713,000		$ 115,784,000
Sales to external customers	412,651,000		359,873,000		318,565,000
Production costs	134,795,000		140,606,000		127,403,000
Depreciation, depletion and amortization	157,078,000	[1]	139,539,000	[1]	127,266,000	[1]
Write-downs of oil and natural gas properties	391,800,000		0		0
Pretax income (loss)	(235,056,000)		173,441,000		179,680,000
Income tax expense (benefit)	(88,612,000)		63,655,000		66,293,000
Results of operations for producing activities	(146,444,000)		109,786,000		113,387,000
Accretion of discount for asset retirement obligations oil and gas	$ 3,300,000		$ 3,600,000		$ 3,200,000

[1] Includes accretion of discount for asset retirement obligations of $3.3 million, $3.6 million and $3.2 million for the years ended December 31, 2012, 2011 and 2010, respectively, as discussed in Note 10.