Quarterly report pursuant to Section 13 or 15(d)

Basis of Presentation and Summary of Significant Accounting Policies (Investment Categories) (Details)

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Basis of Presentation and Summary of Significant Accounting Policies (Investment Categories) (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Summary of investment categories    
Amortized cost $ 110,383 $ 120,758
Gross unrealized gains 2,065 1,564
Gross unrealized losses (44) (26)
Estimated fair value 112,404 122,296
Bank deposits    
Summary of investment categories    
Amortized cost 55,847 40,715
Gross unrealized gains 12 19
Gross unrealized losses (9) 0
Estimated fair value 55,850 40,734
Corporate bonds    
Summary of investment categories    
Amortized cost 30,043 11,031
Gross unrealized gains 2 0
Gross unrealized losses (24) (5)
Estimated fair value 30,021 11,026
Commercial paper    
Summary of investment categories    
Amortized cost 13,948 33,074
Gross unrealized gains 0 2
Gross unrealized losses (3) (9)
Estimated fair value 13,945 33,067
Agency bonds    
Summary of investment categories    
Amortized cost 0 7,294
Gross unrealized gains 0 1
Gross unrealized losses 0 0
Estimated fair value 0 7,295
U.S Government Bonds    
Summary of investment categories    
Amortized cost 8,474 7,508
Gross unrealized gains 0 0
Gross unrealized losses (8) (1)
Estimated fair value 8,466 7,507
Municipal Bonds    
Summary of investment categories    
Amortized cost 1,784 19,624
Gross unrealized gains 0 0
Gross unrealized losses 0 (11)
Estimated fair value 1,784 19,613
Corporate equity securities    
Summary of investment categories    
Amortized cost 287 1,512
Gross unrealized gains 2,051 1,542
Gross unrealized losses 0 0
Estimated fair value $ 2,338 $ 3,054