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Cash and Investments in Marketable Securities - Additional Information (Detail) - USD ($) | 3 Months Ended
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Mar. 31, 2018 | Mar. 31, 2017
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 | $ 11,963,000 | $ 8,823,000
Senior secured notes, maturity date | Oct. 05, 2020 |
Level 1 to level 2 transfers | $ 0 | 0
Level 2 to level 1 transfers | 0 | $ 0
Senior secured notes, principal amount | $ 250,000,000 |
Senior Notes [Member] | Level 3 [Member] |  |
Cash and Investments in Marketable Securities [Line Items] |  |
Debt instrument redemption terms | We may redeem some or all of these notes at a redemption price equal to 104% of the principal amount of the notes if the redemption date is prior to October 5, 2018, 102% of the principal amount of the notes if the redemption date is prior to October 5, 2019, or 100% of the principal amount of the notes if the redemption date is on or after October 5, 2019, plus, in each case, accrued and unpaid interest to the applicable redemption date. |
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 |
Senior Notes [Member] | 7.75% Senior Secured Notes Due October 2020 [Member] | Level 3 [Member] |  |
Cash and Investments in Marketable Securities [Line Items] |  |
Senior secured notes, interest rate | 7.75% |
Senior secured notes, maturity date | Oct. 05, 2020 |
Senior secured notes, principal amount | $ 250,000,000 |
Senior notes fair value | $ 262,000,000 |
Senior Notes [Member] | 7.75% Senior Secured Notes Redemption Date Before October 5, 2018 [Member] | Level 3 [Member] |  |
Cash and Investments in Marketable Securities [Line Items] |  |
Debt redemption price percentage of principal amount | 104.00% |
Senior Notes [Member] | 7.75% Senior Secured Notes Redemption Date Before October 5, 2019 [Member] | Level 3 [Member] |  |
Cash and Investments in Marketable Securities [Line Items] |  |
Debt redemption price percentage of principal amount | 102.00% |
Senior Notes [Member] | 7.75% Senior Secured Notes Redemption Date After October 5, 2019 [Member] | Level 3 [Member] |  |
Cash and Investments in Marketable Securities [Line Items] |  |
Debt redemption price percentage of principal amount | 100.00% |

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- Definition Minimum cash and investments in marketable securities to be maintained. + References No definition available. + Details
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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + 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 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Description of debt redemption features under terms of the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 470 -Section 50 -Paragraph 3 -Subparagraph e -URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823 + 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://www.xbrl.org/2003/role/presentationRef -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://www.xbrl.org/2003/role/presentationRef -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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