Indebtedness - Summary of Debt Facilities (Detail) - USD ($)	Sep. 30, 2018	Dec. 31, 2017	Nov. 30, 2017
Debt Instrument [Line Items]
Other debt facilities		$ 30,390,000
Net carrying value of total debt facilities	$ 415,446,000	431,051,000
2024 Convertible Notes [Member]
Debt Instrument [Line Items]
Par value of the 2024 Notes	570,000,000	570,000,000	$ 570,000,000
Unamortized discount - equity component	(144,999,000)	(158,890,000)	$ (171,800,000)
Unamortized discount - debt issuance costs	(9,555,000)	(10,449,000)
Net carrying value of convertible debt	$ 415,446,000	$ 400,661,000

Including the current and noncurrent portions, carrying amount of debt identified as being convertible into another form of financial instrument (typically the entity's common stock) as of the balance sheet date, which originally required full repayment more than twelve months after issuance or greater than the normal operating cycle of the company.

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