Nature of operations and summary of significant accounting policies Nature of Operations and Summary of Significant Accounting Policies--Cash and Accounts Receivable (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Period within which joint interest accounts receivable are due	30 days
Period within which joint interest accounts receivable are past due	60 days
Bad debt expense	$ (553)	$ 250
Recovery of bad debts	755
Receivables [Abstract]
Joint interests	40,454		31,335
Accrued commodity sales	73,370		60,768
Derivative settlements	156		4,616
Other	2,913		1,392
Allowance for doubtful accounts	(506)		(1,596)
Accounts receivable, net	116,387		96,515
JP Morgan Chase Bank, N.A.
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Cash held at JP Morgan Chase Bank, N. A.	$ 36,695