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Cash and Investments in Marketable Securities - Additional Information (Detail) - USD ($) | 3 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018 | Dec. 31, 2018 | Oct. 05, 2015
Cash and Investments in Marketable Securities [Line Items] |  |  |  |  |  |
Maximum maturity term for debt securities investment |  |  | 2 years |  |  |
Weighted average maturity term for debt securities investment (or less) |  |  | 1 year |  |  |
Available-for-sale securities, sold | $ 0 | $ 11,963,000 | $ 0 | $ 11,963,000 |  |
Level 1 to level 2 transfers | 0 | 0 | 0 | 0 |  |
Level 2 to level 1 transfers | 0 | $ 0 | 0 | $ 0 |  |
Senior secured notes, principal amount | $ 250,000,000.0 |  | $ 250,000,000.0 |  |  |
Senior Notes | 7.75% Senior Secured Notes Due October 2020 |  |  |  |  |  |
Cash and Investments in Marketable Securities [Line Items] |  |  |  |  |  |
Senior secured notes, interest rate |  |  |  |  |  | 7.75%
Minimum cash and investments in marketable securities to be maintained |  |  |  |  |  | $ 60,000,000.0
Senior Notes | 7.75% Senior Secured Notes Due October 2020 | Level 3 |  |  |  |  |  |
Cash and Investments in Marketable Securities [Line Items] |  |  |  |  |  |
Senior secured notes, interest rate | 7.75% |  | 7.75% |  |  |
Senior secured notes, principal amount | $ 250,000,000.0 |  | $ 250,000,000.0 |  |  |
Senior notes, fair value | $ 254,100,000 |  | $ 254,100,000 |  |  |
Senior Notes | 7.75% Senior Secured Notes Redemption Date Before October 5, 2019 | Level 3 |  |  |  |  |  |
Cash and Investments in Marketable Securities [Line Items] |  |  |  |  |  |
Debt redemption price percentage of principal amount |  |  | 102.00% |  |  |
Senior Notes | 7.75% Senior Secured Notes Redemption Date After October 5, 2019 | Level 3 |  |  |  |  |  |
Cash and Investments in Marketable Securities [Line Items] |  |  |  |  |  |
Debt redemption price percentage of principal amount |  |  | 100.00% |  |  |
Minimum |  |  |  |  |  |
Cash and Investments in Marketable Securities [Line Items] |  |  |  |  |  |
Long term investment maturity period |  |  | 1 year |  | 1 year |
Maximum |  |  |  |  |  |
Cash and Investments in Marketable Securities [Line Items] |  |  |  |  |  |
Long term investment maturity period |  |  | 2 years |  | 2 years |

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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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- Definition Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16(a)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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