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Investments Narrative (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Schedule of Investments [Line Items]    
Available-for-sale securities $ 130,785 $ 141,748 [1]
Other-than-temporary impairment on available-for-sale security 1,600 1,300
Gross unrealized losses 816 6,456
Available-for-sale securities, continuous unrealized losses, fair value   124,000
Available-for-sale securities, continuous unrealized losses, 12 months or longer, fair value   92,600
Available-for-sale securities, continuous unrealized accumulated losses, 12 months or longer   5,500
Available-for-sale securities, continuous unrealized losses, 12 months or less, fair value   31,400
Available-for-sale securities, continuous unrealized accumulated losses, 12 months or less   1,000
Trading securities 65,500 32,400
Corporate Debt Securities [Member]    
Schedule of Investments [Line Items]    
Trading securities 45,500 9,000
Mutual Fund [Member]    
Schedule of Investments [Line Items]    
Trading securities 8,900 11,000
Equity Securities [Member]    
Schedule of Investments [Line Items]    
Trading securities $ 7,200 $ 10,500
[1] Investments that calculate NAV as a practical expedient were recategorized and are no longer included within Level 2 of the valuation hierarchy as of December 31, 2015 (see Note (2) for additional information).