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Credit Facilities (Details) (USD $)
1 Months Ended 12 Months Ended
Jan. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2011
Mature In May 2012 [Member]
Dec. 31, 2011
Mature In November 2013 [Member]
Line of Credit Facility [Line Items]          
Total of committed credit facilities with two banks   $ 225,000,000   $ 50,000,000 $ 175,000,000
Committed credit facilities maturity       May 2012 November 2013
Borrowings outstanding   0 0    
Additional borrowings of credit facilities 40,000,000        
Stand-by letters of credit   $ 38,900,000