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 ($)	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2015	Jun. 21, 2013
Debt Instrument [Line Items]
Carrying amount of equity components	$ 22,597,000	$ 22,597,000
Principal amount of liability components	560,613,000	604,276,000
Unamortized debt issuance costs	(4,808,000)	(6,620,000)
Net carrying amount of liability components	560,613,000	604,276,000
Exchangeable Senior Notes 2.375% due 2033 (the 2013 Notes)
Debt Instrument [Line Items]
Carrying amount of equity components	0	0
Principal amount of liability components	0	49,259,000
Unamortized discount - equity components	0	(315,000)
Unamortized cash discount - 2013 Notes	0	(74,000)
Unamortized debt issuance costs	(10,000)		$ (35,000)		$ (1,672,000)
Exchangeable Senior Notes 3.125% due 2035 (the 2015 Notes)
Debt Instrument [Line Items]
Carrying amount of equity components	22,597,000	22,597,000
Principal amount of liability components	575,000,000	575,000,000
Unamortized discount - equity components	$ (9,579,000)	$ (12,974,000)
Unamortized debt issuance costs				$ (11,992,000)