Condensed Consolidating Financial Information - Schedule of Condensed Consolidating Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
ASSETS
Cash and cash equivalents	$ 125,440	$ 213,447	$ 280,303	$ 470,872
Restricted cash	986,210	0
Short-term investments	0	25,891
Accounts receivable — oil and gas revenues	155,068	110,341
Accounts receivable — joint interest partners	120,058	99,194
Accounts receivable – from affiliates
Inventory	18,358	20,707
Prepaid expenses	7,440	1,770
Advances to joint interest partners	1,170	1,985
Derivative instruments	374	19,016
Deferred income taxes	8,683
Other current assets	473	335
Total current assets	1,423,274	492,686
Property, plant and equipment
Oil and gas properties (successful efforts method)	3,044,515	2,348,128
Other property and equipment	157,926	49,732
Less: accumulated depreciation, depletion, amortization and impairment	(589,173)	(391,260)
Total property, plant and equipment, net	2,613,268	2,006,600
Derivative instruments	3,405	4,981
Deferred income taxes
Deferred costs and other assets	43,436	24,527
Total assets	4,083,383	2,528,794
LIABILITIES AND STOCKHOLDERS’ EQUITY
Accounts payable	39,468	12,491
Accounts payable – from affiliates
Advances from joint interest partners	13,211	21,176
Revenues and production taxes payable	133,083	71,553
Accrued liabilities	198,493	189,863
Accrued interest payable	22,873	30,096
Derivative instruments	17,060	1,048
Deferred income taxes	0	4,558
Total current liabilities	424,188	330,785
Long-term debt	2,360,000	1,200,000
Asset retirement obligations	26,999	22,956
Derivative instruments	852	380
Deferred income taxes	293,156	177,671
Other liabilities	2,310	1,997
Total liabilities	3,107,505	1,733,789
Stockholders’ equity
Common stock, $0.01 par value; 300,000,000 shares authorized; 93,854,867 issued and 93,690,494 outstanding at September 30, 2013; 93,432,712 issued and 93,303,298 outstanding at December 31, 2012	926	925
Treasury stock, at cost; 164,373 and 129,414 shares at September 30, 2013 and December 31, 2012, respectively	(5,220)	(3,796)
Additional paid-in-capital	666,770	657,943
Retained earnings	313,402	139,933
Total stockholders’ equity	975,878	795,005
Total liabilities and stockholders’ equity	4,083,383	2,528,794
Parent/Issuer [Member]
ASSETS
Cash and cash equivalents	32,955	133,797	231,170	443,482
Restricted cash	986,210
Short-term investments		25,891
Accounts receivable – from affiliates	770	310
Prepaid expenses	477	313
Other current assets	2	235
Total current assets	1,020,414	160,546
Property, plant and equipment
Investments in and advances to subsidiaries	2,082,828	1,807,010
Deferred income taxes	69,795	42,746
Deferred costs and other assets	34,128	20,748
Total assets	3,207,165	2,031,050
LIABILITIES AND STOCKHOLDERS’ EQUITY
Accounts payable		9
Accounts payable – from affiliates	8,432	5,845
Accrued liabilities	46	100
Accrued interest payable	22,809	30,091
Total current liabilities	31,287	36,045
Long-term debt	2,200,000	1,200,000
Total liabilities	2,231,287	1,236,045
Stockholders’ equity
Common stock, $0.01 par value; 300,000,000 shares authorized; 93,854,867 issued and 93,690,494 outstanding at September 30, 2013; 93,432,712 issued and 93,303,298 outstanding at December 31, 2012	926	925
Treasury stock, at cost; 164,373 and 129,414 shares at September 30, 2013 and December 31, 2012, respectively	(5,220)	(3,796)
Additional paid-in-capital	666,770	657,943
Retained earnings	313,402	139,933
Total stockholders’ equity	975,878	795,005
Total liabilities and stockholders’ equity	3,207,165	2,031,050
Combined Guarantor Subsidiaries [Member]
ASSETS
Cash and cash equivalents	92,485	79,650	49,133	27,390
Accounts receivable — oil and gas revenues	155,068	110,341
Accounts receivable — joint interest partners	120,058	99,194
Accounts receivable – from affiliates	8,432	5,845
Inventory	18,358	20,707
Prepaid expenses	6,963	1,457
Advances to joint interest partners	1,170	1,985
Derivative instruments	374	19,016
Deferred income taxes	8,683
Other current assets	471	100
Total current assets	412,062	338,295
Property, plant and equipment
Oil and gas properties (successful efforts method)	3,044,515	2,348,128
Other property and equipment	157,926	49,732
Less: accumulated depreciation, depletion, amortization and impairment	(589,173)	(391,260)
Total property, plant and equipment, net	2,613,268	2,006,600
Derivative instruments	3,405	4,981
Deferred costs and other assets	9,308	3,779
Total assets	3,038,043	2,353,655
LIABILITIES AND STOCKHOLDERS’ EQUITY
Accounts payable	39,468	12,482
Accounts payable – from affiliates	770	310
Advances from joint interest partners	13,211	21,176
Revenues and production taxes payable	133,083	71,553
Accrued liabilities	198,447	189,763
Accrued interest payable	64	5
Derivative instruments	17,060	1,048
Deferred income taxes		4,558
Total current liabilities	402,103	300,895
Long-term debt	160,000
Asset retirement obligations	26,999	22,956
Derivative instruments	852	380
Deferred income taxes	362,951	220,417
Other liabilities	2,310	1,997
Total liabilities	955,215	546,645
Stockholders’ equity
Capital contributions from affiliates	1,643,729	1,586,780
Additional paid-in-capital	8,743	8,743
Retained earnings	430,356	211,487
Total stockholders’ equity	2,082,828	1,807,010
Total liabilities and stockholders’ equity	3,038,043	2,353,655
Intercompany Eliminations [Member]
ASSETS
Accounts receivable – from affiliates	(9,202)	(6,155)
Total current assets	(9,202)	(6,155)
Property, plant and equipment
Investments in and advances to subsidiaries	(2,082,828)	(1,807,010)
Deferred income taxes	(69,795)	(42,746)
Total assets	(2,161,825)	(1,855,911)
LIABILITIES AND STOCKHOLDERS’ EQUITY
Accounts payable – from affiliates	(9,202)	(6,155)
Total current liabilities	(9,202)	(6,155)
Deferred income taxes	(69,795)	(42,746)
Total liabilities	(78,997)	(48,901)
Stockholders’ equity
Capital contributions from affiliates	(1,643,729)	(1,586,780)
Additional paid-in-capital	(8,743)	(8,743)
Retained earnings	(430,356)	(211,487)
Total stockholders’ equity	(2,082,828)	(1,807,010)
Total liabilities and stockholders’ equity	$ (2,161,825)	$ (1,855,911)