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Financial Instruments (Tables)
 12 Months Ended
Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]  
Schedule of Realized Gain (Loss)
 
 
Year Ended December 31,
 
 
2017
 
2016
 
2015
Proceeds from sales and maturities of securities
 
$
—

 
$
663

 
$
49

Gross realized gains
 
—

 
54

 
4