Fair Value Measurements - Additional Information (Detail) - USD ($)
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6 Months Ended |
12 Months Ended |
Jan. 17, 2017 |
Jul. 18, 2013 |
Jun. 30, 2018 |
Jun. 30, 2017 |
Dec. 31, 2017 |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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| Debt issuance costs paid |
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$ 5,902,000
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| 6.625% Senior Notes Due 2021 [Member] | Senior Notes [Member] |
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| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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| Interest rate |
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6.625%
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6.625%
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6.625%
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| Due date |
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Jul. 15, 2021
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Jul. 15, 2021
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Jul. 15, 2021
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| Aggregate principal amount |
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$ 225,000,000
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| Debt issuance costs paid |
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5,039,000
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| Original debt issue discount |
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$ 1,125,000
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| 5.0% Exchangeable Senior Notes due March 2021 [Member] | Exchangeable Senior Notes [Member] |
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| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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| Interest rate |
5.00%
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5.00%
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5.00%
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| Due date |
Mar. 01, 2021
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Mar. 01, 2021
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Mar. 01, 2021
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| Aggregate principal amount |
$ 86,250,000
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| Debt issuance costs paid |
$ 5,902,000
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$ 5,902,000
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| Other Asset Class [Member] | Fair Value, Measurements, Nonrecurring [Member] |
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| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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| Assets fair value on non-recurring basis |
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0
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0
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| Liabilities fair value on non-recurring basis |
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$ 0
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$ 0
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