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
Mar. 31, 2017
Dec. 31, 2016
Summary of investment categories    
Amortized cost $ 146,633 $ 120,758
Gross unrealized gains 2,139 1,564
Gross unrealized losses (39) (26)
Estimated fair value 148,733 122,296
Bank deposits    
Summary of investment categories    
Amortized cost 61,605 40,715
Gross unrealized gains 23 19
Gross unrealized losses (10) 0
Estimated fair value 61,618 40,734
Corporate bonds    
Summary of investment categories    
Amortized cost 32,237 11,031
Gross unrealized gains 2 0
Gross unrealized losses (17) (5)
Estimated fair value 32,222 11,026
Commercial paper    
Summary of investment categories    
Amortized cost 23,328 33,074
Gross unrealized gains 1 2
Gross unrealized losses (5) (9)
Estimated fair value 23,324 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 11,020 7,508
Gross unrealized gains 0 0
Gross unrealized losses (6) (1)
Estimated fair value 11,014 7,507
Municipal Bonds    
Summary of investment categories    
Amortized cost 18,143 19,624
Gross unrealized gains 2 0
Gross unrealized losses (1) (11)
Estimated fair value 18,144 19,613
Corporate equity securities    
Summary of investment categories    
Amortized cost 300 1,512
Gross unrealized gains 2,111 1,542
Gross unrealized losses 0 0
Estimated fair value $ 2,411 $ 3,054