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Marketable Securities (Narrative) (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended 12 Months Ended 1 Months Ended 3 Months Ended 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2013
Trading Securities [Member]
Dec. 31, 2012
Trading Securities [Member]
Nov. 30, 2012
SIV Securities [Member]
Dec. 31, 2012
SIV Securities [Member]
Sep. 30, 2012
SIV Securities [Member]
Jun. 30, 2012
SIV Securities [Member]
Mar. 31, 2012
SIV Securities [Member]
Dec. 31, 2012
SIV Securities [Member]
Dec. 31, 2011
SIV Securities [Member]
Dec. 31, 2013
U.S. Government Agency And Commercial Paper Securities [Member]
Dec. 31, 2012
U.S. Government Agency And Commercial Paper Securities [Member]
Schedule of Investments [Line Items]                              
Net unrealized holding gains $ 1,386 $ 2,829 $ 2,574 $ 1,339                      
Unrealized holding gains during the period, income tax expense 430 1,554                          
Available-for-sale securities, gross realized gains 1,236                            
Available-for-sale securities, gross realized losses 772                            
Trading Securities 4,849 5,909     4,849 4,706                  
Trading Securities, Realized Gain         143 1,004           13,240 3,390    
Gains From Cash Payments Received From SIV Securities                       6,776 10,614    
Unrealized Gain (Loss) on Securities                       1,142 (7,224)    
Trading Securities, Realized Loss             5,322 5,897 3,389 1,072 2,882        
Securities owned $ 21,133 $ 20,088                       $ 21,133 $ 20,088