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Marketable Securities Marketable Security - Narrative (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
security
Dec. 31, 2016
USD ($)
segment
security
Investments, Debt and Equity Securities [Abstract]    
Available for sale securities in unrealized loss positions 0 0
Number of debt securities in unrealized loss position | security 38 13
Aggregate unrealized loss on debt securities $ 315 $ 6
Fair value of debt securities $ 299,200 35,700
Marketable securities maturity term 1 year  
Long-term investments maturity term 2 years  
Investments $ 176,978 $ 0