ADDITIONAL BALANCE SHEET INFORMATION (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Accounts receivable, net
Trade accounts	$ 167,405	$ 165,070
Joint interest accounts	4,826	5,659
Other accounts	1,555	2,817
Accounts receivable, gross	173,786	173,546
Allowance for doubtful accounts	(1,125)	(1,127)
Accounts receivable, net	172,661	172,419
Inventories
Natural gas in storage	3,425	7,494
Tubular goods and well equipment	8,011	6,392
Other accounts	(158)	287
Inventory, net	11,278	14,173
Other current assets
Prepaid balances and other	1,174	2,158
Other assets current	1,174	2,158
Other assets
Deferred compensation plan	11,372	10,608
Debt issuance costs	16,523	17,420
Equity method investment	8,211	6,915
Other accounts	78	83
Other assets	36,184	35,026
Accounts payable
Trade accounts	11,006	14,037
Natural gas purchases	2,331	4,892
Royalty and other owners	71,028	66,321
Accrued capital costs	154,498	164,862
Taxes other than income	13,504	10,224
Drilling advances	40,607	44,203
Producer gas imbalances	1,628	1,602
Other accounts	5,131	6,339
Accounts payable current	299,733	312,480
Accrued liabilities
Employee benefits	17,346	16,011
Postretirement benefits	1,304	1,304
Taxes other than income	8,959	8,735
Interest payable	12,650	22,329
Other accounts	2,294	1,218
Accrued liabilities	42,553	49,597
Other liabilities
Deferred compensation plan	29,429	23,893
Other accounts	12,793	16,282
Other liabilities	$ 42,222	$ 40,175