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Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2013
Dec. 31, 2012
Deferred tax assets:    
Deferred compensation $ 9,128 $ 6,192
Net operating loss carryforward 57,266 56,402
Total current 57,191 16,049
Deferred compensation 91,094 72,904
Equity compensation 22,800 23,363
AMT credits and other credits 4,122 2,761
Valuation allowance (14,781) (9,052)
Total non-current 247,743 204,259
Deferred tax liabilities:    
Total current (5,776) (53,973)
Depreciation, depletion and investments (1,010,757) (894,031)
Total non-current (1,019,724) (902,561)
Net deferred tax liability (720,566) (736,226)
Current
    
Deferred tax assets:    
Portion of asset retirement obligation 1,854 961
Cumulative unrealized mark-to-market (gain) / loss 15,193  
Net operating loss carryforward 23,079  
Other 7,937 8,896
Non-current
    
Deferred tax assets:    
Portion of asset retirement obligation 86,126 56,764
Cumulative unrealized mark-to-market (gain) / loss   (262)
Other 1,116 1,379
Current
    
Deferred tax liabilities:    
Net unrealized gain in AOCI related to hedge derivatives (3,987) (49,124)
Other (1,789) (2,004)
Cumulative unrealized mark-to-market gain   (2,845)
Non-current
    
Deferred tax liabilities:    
Net unrealized gain in AOCI related to hedge derivatives   (4,522)
Other (2,542) (4,008)
Cumulative unrealized mark-to-market gain $ (6,424)