Quarterly report pursuant to Section 13 or 15(d)

Summary of Incentive Compensation Expense (Detail)

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Summary of Incentive Compensation Expense (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]    
Share-based compensation charged to operating income $ 2,255 $ 2,659
Cash-based incentive compensation charged to operating income 4,923 3,778
Total incentive compensation charged to operating income 7,178 6,437
Lease Operating Expense
   
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]    
Cash-based incentive compensation charged to operating income 1,393 1,900
General and Administrative Expense
   
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]    
Share-based compensation charged to operating income 2,255 2,659
Cash-based incentive compensation charged to operating income $ 3,530 $ 1,878