Exchangeable Senior Notes - Schedule of Information about Total Carrying Amounts of Equity Components, Principal Amounts of Liability Components, Unamortized Discounts and Net Carrying Amounts for Notes (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2015
Debt Instrument [Line Items]
Principal amount of liability component	$ 565,920	$ 562,374
Exchangeable Senior Notes 3.125% due 2035 (the 2015 Notes)
Debt Instrument [Line Items]
Principal amount of liability component	575,000
Unamortized debt issuance costs			$ (11,992)
Senior Notes | Exchangeable Senior Notes 3.125% due 2035 (the 2015 Notes)
Debt Instrument [Line Items]
Carrying amount of equity component	22,597	22,597
Principal amount of liability component	575,000	575,000
Unamortized discount - equity component	(6,070)	(8,417)
Unamortized debt issuance costs	(3,010)	(4,209)
Net carrying amount of liability components	$ 565,920	$ 562,374