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Investments - Summary of Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	$ 92,290	$ 89,683
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(380)	(412)
Estimated Fair Value	91,910	89,271
Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	4,915	4,915
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(1)	(3)
Estimated Fair Value	4,914	4,912
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	3,984
Gross Unrealized Gains	0
Estimated Fair Value	3,984
Municipal Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	83,391	84,768
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(379)	(409)
Estimated Fair Value	$ 83,012	$ 84,359

X

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Amount of investment in debt and equity securities categorized neither as trading securities nor held-to-maturity securities and intended be sold or mature one year or operating cycle, if longer.

+ 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)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26626-111562

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