Annual report pursuant to Section 13 and 15(d)

Schedule of Changes in Standardized Measure (Detail)

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Schedule of Changes in Standardized Measure (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Reserve Quantities [Line Items]      
Changes in Standardized Measure Standardized measure, beginning of year $ 1,846.4 $ 2,006.4 $ 1,179.1
Sales and transfers of oil and gas produced, net of production costs (686.1) (620.4) (729.6)
Net changes in price, net of future production costs (65.2) (224.3) 634.2
Extensions and discoveries, net of future production and development costs 393.8 181.9 219.9
Changes in estimated future development costs (91.1) (103.3) (4.6)
Previously estimated development costs incurred 262.1 332.9 173.9
Revisions of quantity estimates (91.6) (128.1) 205.0
Accretion of discount 202.2 231.1 135.8
Net change in income taxes 56.6 99.7 (398.2)
Purchases of reserves in-place 79.6 270.2 483.3
Sales of reserves in-place (53.1) (16.1)  
Changes in production rates due to timing and other (179.0) (183.6) 107.6
Net increase (decrease) in standardized measure (171.8) (160.0) 827.3
Changes in Standardized measure, end of year $ 1,674.6 $ 1,846.4 $ 2,006.4