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