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Cash Equivalents and Investments - Schedule of Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Short-Term	$ 10,844	$ 10,292
Long-Term	90,299	80,682
Municipal Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Short-Term	8,614	8,312
Long-Term	49,841	46,195
Federal Home Loan Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Short-Term	1,250	1,000
Long-Term	35,793	30,322
Certificates of Deposit [Member]
Schedule of Held-to-maturity Securities [Line Items]
Short-Term	980	980
Long-Term	$ 4,665	$ 4,165

X

- Definition

Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity, expected to be converted to cash, sold or exchanged within one year or the normal operating cycle.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

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- Definition

Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity, not expected to be converted to cash, sold or exchanged within one year or the normal 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 (c)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

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-Name Accounting Standards Codification
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-Section 45
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