ADDITIONAL BALANCE SHEET INFORMATION (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Accounts receivable, net
Trade accounts	$ 193,695	$ 165,070
Joint interest accounts	6,694	5,659
Other accounts	6,260	2,817
Accounts receivable, gross	206,649	173,546
Allowance for doubtful accounts	(1,679)	(1,127)
Accounts receivable, net	204,970	172,419
Inventories
Natural gas in storage	8,629	7,494
Tubular goods and well equipment	9,274	6,392
Other accounts	373	287
Inventory, net	18,276	14,173
Other current assets
Prepaid balances and other	4,889	2,158
Other assets current	4,889	2,158
Other assets
Deferred compensation plan	11,416	10,608
Debt issuance costs	15,578	17,420
Equity method investment	11,501	6,915
Other accounts	78	83
Other assets	38,573	35,026
Accounts payable
Trade accounts	19,134	14,037
Natural gas purchases	6,335	4,892
Royalty and other owners	81,743	66,321
Accrued capital costs	184,891	164,862
Taxes other than income	6,947	10,224
Drilling advances	51,026	44,203
Producer gas imbalances	1,368	1,602
Other accounts	5,407	6,339
Accounts payable current	356,851	312,480
Accrued liabilities
Employee benefits	20,779	16,011
Postretirement benefits	1,304	1,304
Taxes other than income	11,374	8,735
Interest payable	22,128	22,329
Derivative instruments		192
Other accounts	2,986	1,218
Accrued liabilities	58,571	49,789
Other liabilities
Deferred compensation plan	30,385	23,893
Other accounts	15,723	16,282
Other liabilities	$ 46,108	$ 40,175