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Employee Benefit Plans - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Defined Benefit Plan Disclosure [Line Items]      
Employee benefits expenses $ 11.8 $ 12.6 $ 10.9
Maximum 401(k) plan contribution rates as a percentage of employees earnings 100.00%    
Maximum employers 401(k) contribution to employee 6.00%    
Postretirement Medical Plan
     
Defined Benefit Plan Disclosure [Line Items]      
Net periodic benefit cost 0.1 0.1 0.1
Retirement Plan
     
Defined Benefit Plan Disclosure [Line Items]      
Lump-sum cash payments to terminate Non-Qualified plan     10.4
Net periodic benefit cost   0.1 0.8
Compensation and benefits expenses related to settlement of all Non-Qualified Plan liabilities   $ 0.2