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Borrowing Arrangements (Details Textual) (USD $)
 1 Months Ended 3 Months Ended
Sep. 27, 2010
Mar. 31, 2013
Dec. 31, 2012
Borrowing Arrangements (Additional Textual) [Abstract]      
Primary debt arrangements   2  
Unsecured credit facility   $ 275,000,000  
Group of participating banks   7  
Approximately available funds under credit facility   47,600,000  
Maturity date of credit facility   2015-06  
Outstanding balance under the credit facility   223,000,000 208,900,000
2010 Convertible Senior Subordinated Notes [Member]
      
Borrowing Arrangements (Textual) [Abstract]      
Convertible Senior Subordinated Notes 130,000,000 130,000,000  
Interest rate on Notes 4.875%    
Notes payable terms Semi-annually in arrears on April 1 and October 1    
Maturity date of Notes Oct. 01, 2015    
Amortized discount at annual effective rate 7.50%    
Discount, period of issuance 5 years    
Remaining period for amortization of discount   30 months  
Principal amount of notes   130,000,000 130,000,000
2006 Convertible Senior Subordinated Notes [Member]
      
Borrowing Arrangements (Textual) [Abstract]      
Interest rate on Notes   3.125%  
Notes payable terms   semi-annually in arrears on each June 1 and December 1  
Maturity date of Notes   Jun. 01, 2026  
Principal amount of notes   750,000 750,000
Letter of Credit and Performance Guarantee [Member]
      
Borrowing Arrangements (Textual) [Abstract]      
Outstanding letters of credit and performance guarantees   $ 4,400,000