Marketable Securities - Summary of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Summary of available-for-sale securities
Available-for-sale securities, Cost	$ 21,519	$ 26,773
Available-for-sale securities, Fair Value	21,494	26,675
Level 1 \| Certificates of deposit
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	1,295	1,868
Unrecognized losses for more than 12 months, Cost	0	0
Unrecognized gains, Cost	3,494	3,320
Available-for-sale securities, Cost	4,789	5,188
Unrecognized losses for less than 12 months, Fair Value	1,292	1,863
Unrecognized losses for more than 12 months, Fair Value	0	0
Unrecognized gains, Fair value	3,500	3,329
Available-for-sale securities, Fair Value	4,792	5,192
Level 2 \| U.S. Treasury and agency notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	1,051	947
Unrecognized losses for more than 12 months, Cost	0	0
Unrecognized gains, Cost	120	857
Available-for-sale securities, Cost	1,171	1,804
Unrecognized losses for less than 12 months, Fair Value	1,050	946
Unrecognized losses for more than 12 months, Fair Value	0	0
Unrecognized gains, Fair value	120	859
Available-for-sale securities, Fair Value	1,170	1,805
Level 2 \| Corporate notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	7,863	11,658
Unrecognized losses for more than 12 months, Cost	1,681	1,053
Unrecognized gains, Cost	2,955	3,431
Available-for-sale securities, Cost	12,499	16,142
Unrecognized losses for less than 12 months, Fair Value	7,849	11,592
Unrecognized losses for more than 12 months, Fair Value	1,677	1,042
Unrecognized gains, Fair value	2,957	3,435
Available-for-sale securities, Fair Value	12,483	16,069
Level 2 \| Municipal notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	1,428	2,556
Unrecognized losses for more than 12 months, Cost	600	271
Unrecognized gains, Cost	1,032	812
Available-for-sale securities, Cost	3,060	3,639
Unrecognized losses for less than 12 months, Fair Value	1,419	2,526
Unrecognized losses for more than 12 months, Fair Value	596	269
Unrecognized gains, Fair value	1,034	814
Available-for-sale securities, Fair Value	$ 3,049	$ 3,609

Available for sale securities, unrealized gain (loss), fair value.

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Amount before tax of unrealized gain (loss) in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

No definition available.

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- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

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-Publisher FASB
-Name Accounting Standards Codification
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This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

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