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		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Cash and accounts receivable
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	$ 39	$ 268	$ (514)	$ 519
Components of accounts receivable
Joint interests	32,971		32,971		31,335
Accrued commodity sales	77,334		77,334		60,768
Derivative settlements	0		0		4,616
Production tax credit	2,031		2,031		694
Other	1,354		1,354		698
Allowance for doubtful accounts	(325)		(325)		(1,596)
Accounts receivable, net	113,365		113,365		96,515
JP Morgan Chase Bank, N.A.
Cash and accounts receivable
Cash held at JP Morgan Chase Bank, N. A.	$ 31,539		$ 31,539