Annual report pursuant to Section 13 and 15(d)

Summary of Incentive Compensation Expense (Detail)

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Summary of Incentive Compensation Expense (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]      
Share-based compensation charged to operating income $ 12,398 $ 9,710 $ 5,533
Cash-based incentive compensation charged to operating income 10,345 15,913 10,606
Total incentive compensation charged to operating income 22,743 25,623 16,139
Lease Operating Expense
      
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]      
Share-based compensation charged to operating income   466 748
Cash-based incentive compensation charged to operating income 3,787 3,700 2,067
General and Administrative Expense
      
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]      
Share-based compensation charged to operating income 12,398 9,244 4,785
Cash-based incentive compensation charged to operating income $ 6,558 $ 12,213 $ 8,539