Annual report pursuant to Section 13 and 15(d)

Commitments - Additional Information (Detail)

v2.4.0.6
Commitments - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Operating Leased Assets [Line Items]      
Minimum future lease payments due under noncancelable operating leases, 2013 $ 1.2    
Minimum future lease payments due under noncancelable operating leases, 2014 1.3    
Minimum future lease payments due under noncancelable operating leases, 2015 1.3    
Minimum future lease payments due under noncancelable operating leases, thereafter 9.3    
Total rent expense 1.7 1.9 2.0
Additional security requirements for 2013 9.0    
Additional security requirements for 2014 9.0    
Additional security requirements for 2015 9.0    
Additional security requirements for 2016 6.0    
Additional security requirements 2017 to 2023 24.0    
Security amount requirement 46.0    
Total security requirement 103.0    
Minimum
      
Operating Leased Assets [Line Items]      
Current security requirement 74.0    
Maximum
      
Operating Leased Assets [Line Items]      
Current security requirement 94.0    
Drilling Rig Commitments
      
Operating Leased Assets [Line Items]      
Minimum future lease payments due under noncancelable operating leases, 2013 $ 36.5