Long-Term Debt - Summary of Components of Long-Term Debt Obligations (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total debt	$ 316,250,000	$ 400,000,000
Long-term debt, net	265,571,000	396,284,000
Long-term debt, current portion	0
Credit facility
Debt Instrument [Line Items]
Total debt	0	100,000,000
Convertible Debt
Debt Instrument [Line Items]
Less: deferred debt discount	(1,175,000)	0
Convertible Debt \| Convertible Note Due 2023
Debt Instrument [Line Items]
Total debt	316,250,000	0
Less: deferred debt discount	(43,998,000)	0
Less: deferred debt issue costs	(6,681,000)	(3,716,000)
Additional paid-in capital	35,306,000	0
Equity component, net	34,131,000	0
Convertible Debt \| 6% senior notes due 2022
Debt Instrument [Line Items]
Total debt	$ 0	$ 300,000,000

Excess of issue price over par or stated value of the entity's capital stock and amounts received from other transactions involving the entity's stock or stockholders. Includes adjustments to additional paid in capital. Some examples of such adjustments include recording the issuance of debt with a beneficial conversion feature and certain tax consequences of equity instruments awarded to employees. Use this element for the aggregate amount of additional paid-in capital associated with common and preferred stock. For additional paid-in capital associated with only common stock, use the element additional paid in capital, common stock. For additional paid-in capital associated with only preferred stock, use the element additional paid in capital, preferred stock.

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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The carrying amount of the equity component of convertible debt which may be settled in cash upon conversion.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount, after accumulated amortization, of debt discount.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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