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	9 Months Ended
	Sep. 30, 2014	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
Components of accounts receivable
Joint interests	$ 36,644	$ 31,335
Accrued commodity sales	65,304	60,768
Derivative settlements	1,041	4,616
Production tax credit	2,703	694
Other	6,192	698
Allowance for doubtful accounts	(311)	(1,596)
Accounts receivable, net	111,573	96,515
JP Morgan Chase Bank, N.A.
Cash and accounts receivable
Cash held at JP Morgan Chase Bank, N. A.	$ 17,457