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Convertible Senior Notes - Debt Net of Discounts and Deferred Financing Costs (Detail) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Debt Disclosure [Abstract]    
Principal $ 517,500 $ 517,500
Deferred financing costs (7,290) (8,326)
Debt discount, net (106,621) (120,678)
Net carrying amount $ 403,589 $ 388,496