Income resulting from oil and natural gas producing activites (Details 3) (USD $)	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Revenues:
Sales to affiliates	$ 45,099,000		$ 35,966,000		$ 93,713,000
Sales to external customers	497,018,000		379,647,000		348,428,000
Realized gain on commodity derivatives	173,000		33,628,000		9,618,000
Unrealized (gain) loss on commodity derivatives	(6,267,000)		(624,000)		1,827,000
Production costs	144,136,000		134,795,000		140,606,000
Depreciation, depletion and amortization	182,352,000	[1]	157,078,000	[1]	139,539,000	[1]
Write-downs of oil and natural gas properties	0		391,800,000		0
Pretax income (loss)	209,535,000		(235,056,000)		173,441,000
Income tax expense (benefit)	75,836,000		(88,612,000)		63,655,000
Results of operations for producing activities	133,699,000		(146,444,000)		109,786,000
Accretion of discount for asset retirement obligations oil and gas	$ 3,600,000		$ 3,300,000		$ 3,600,000

[1] Includes accretion of discount for asset retirement obligations of $3.6 million, $3.3 million and $3.6 million for the years ended December 31, 2013, 2012 and 2011, respectively, as discussed in Note 10.