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Basis of Presentation and Summary of Significant Accounting Policies - Investment Categories (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Summary of investment categories
Amortized cost	$ 212,120	$ 179,528
Gross unrealized gains	1,582	1,695
Gross unrealized losses	(373)	(182)
Estimated fair value	213,329	181,041
Bank deposits
Summary of investment categories
Amortized cost	94,855	80,095
Gross unrealized gains	5	6
Gross unrealized losses	(97)	(42)
Estimated fair value	94,763	80,059
Corporate bonds
Summary of investment categories
Amortized cost	73,129	55,335
Gross unrealized gains	0	0
Gross unrealized losses	(216)	(96)
Estimated fair value	72,913	55,239
Commercial paper
Summary of investment categories
Amortized cost	29,977	27,933
Gross unrealized gains	0	0
Gross unrealized losses	(26)	(20)
Estimated fair value	29,951	27,913
U.S. Government bonds
Summary of investment categories
Amortized cost	11,964	8,939
Gross unrealized gains	0	0
Gross unrealized losses	(22)	(10)
Estimated fair value	11,942	8,929
Agency bonds
Summary of investment categories
Amortized cost	0	4,991
Gross unrealized gains	0	0
Gross unrealized losses	0	(1)
Estimated fair value	0	4,990
Municipal bonds
Summary of investment categories
Amortized cost	2,014	2,028
Gross unrealized gains	0	0
Gross unrealized losses	(12)	(13)
Estimated fair value	2,002	2,015
Corporate equity securities
Summary of investment categories
Amortized cost	181	207
Gross unrealized gains	1,577	1,689
Gross unrealized losses	0	0
Estimated fair value	$ 1,758	$ 1,896

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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