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Cash and Investments in Marketable Securities - Additional Information (Detail) - USD ($) |  | 12 Months Ended
--- | --- | ---
Oct. 05, 2015 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Cash and Investments in Marketable Securities [Line Items] |  |  |  |
Maximum maturity term for debt securities investment |  | Two years or less |  |
Weighted average maturity term for debt securities investment |  | One year or less |  |
Available-for-sale securities, sold |  | $ 12,000,000 | $ 37,500,000 | $ 5,000,000
Senior secured notes, maturity date |  | Oct. 05, 2020 |  |
Level 1 to level 2 transfers |  | $ 0 | 0 |
Level 2 to level 1 transfers |  | 0 | 0 |
Letter of credit |  | $ 6,600,000 | $ 1,100,000 |
Senior Notes [Member] | 7.75% Senior Secured Notes Due October 2020 [Member] |  |  |  |
Cash and Investments in Marketable Securities [Line Items] |  |  |  |
Senior secured notes, interest rate | 7.75% |  |  |
Senior secured notes, maturity date | Oct. 05, 2020 |  |  |
Minimum cash and investments in marketable securities to be maintained | $ 60,000,000 |  |  |

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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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