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Investments Narrative (Details) - USD ($)
$ in Thousands
 3 Months Ended
Sep. 30, 2015
Dec. 31, 2015
Dec. 31, 2014
Schedule of Investments [Line Items]      
Available-for-sale securities   $ 141,748 $ 143,190
Gross unrealized losses   6,456 3,104
Available-for-sale securities, continuous unrealized losses, fair value   124,000 124,800
Available-for-sale securities, continuous unrealized losses, 12 months or longer, fair value   92,600 4,500
Available-for-sale securities, continuous unrealized accumulated losses, 12 months or longer   5,500 900
Available-for-sale securities, continuous unrealized losses, 12 months or less, fair value   31,400 120,300
Available-for-sale securities, continuous unrealized accumulated losses, 12 months or less   1,000 2,200
Other-than-temporary impairment on available-for-sale security $ 1,300    
Trading securities   32,400 38,900
Investments in sponsored mutual funds [Member]      
Schedule of Investments [Line Items]      
Trading securities   11,000 8,100
Stocks of large U.S. and international companies [Member]      
Schedule of Investments [Line Items]      
Trading securities   10,500 21,300
Corporate Debt Securities [Member]      
Schedule of Investments [Line Items]      
Trading securities   $ 9,000 $ 7,500