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Derivative Activities - Fair Value of Derivative Contracts, by Consolidated Balance Sheets (Detail) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2013
Dec. 31, 2012
Derivatives Fair Value [Line Items]    
Assets Carrying Value $ 45,776 $ 152,114
Liabilities Carrying Value (14,316) (12,963)
Net Carrying Value 31,460 139,151
Assets Carrying Value 60,564 15,086
Liabilities Carrying Value (26,031) (8,904)
Net Carrying Value 34,533 6,182
Swaps
    
Derivatives Fair Value [Line Items]    
Assets Carrying Value 7,091 [1] 22,236 [1]
Liabilities Carrying Value (3,960) [1] (3,242) [1]
Net Carrying Value 3,131 [1] 18,994 [1]
Collars
    
Derivatives Fair Value [Line Items]    
Assets Carrying Value 38,685 [1] 129,878 [1]
Liabilities Carrying Value (10,356) [1] (9,721) [1]
Net Carrying Value 28,329 [1] 120,157 [1]
Assets Carrying Value 42,943 [1] 857 [1]
Liabilities Carrying Value (2,959) [1]  
Net Carrying Value 39,984 [1] 857 [1]
Sold Swap
    
Derivatives Fair Value [Line Items]    
Assets Carrying Value 16,159 [1] 7,316 [1]
Liabilities Carrying Value (23,072) [1] (8,904) [1]
Net Carrying Value (6,913) [1] (1,588) [1]
Re-purchased swaps
    
Derivatives Fair Value [Line Items]    
Assets Carrying Value 1,462 [1] 5,920 [1]
Net Carrying Value 1,462 [1] 5,920 [1]
Basis Swap
    
Derivatives Fair Value [Line Items]    
Assets Carrying Value   993 [1]
Net Carrying Value   $ 993 [1]
[1] Included in unrealized derivatives in the accompanying consolidated balance sheets. See additional discussion above regarding the discontinuance of hedge accounting.