Contractual Coupon Interest Expense (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt Instrument [Line Items]
Contractual coupon interest expense	$ 0	$ 379	$ 470	$ 1,194
Accretion of discount and fees on the liability component	$ 0	$ 374	$ 461	$ 1,159

Amount recognized for the passage of time, typically for liabilities, that have been discounted to their net present values. Excludes accretion associated with asset retirement obligations.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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- Definition

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.

+ References

No definition available.

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- Definition

Amount of the cost of borrowed funds accounted for as interest expense for debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
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