Exchangeable Senior Notes - Summary of Amount of Interest Cost Recognized Relating to Contractual Interest Rates and Amortization of Discounts on Liability Components of Notes (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Debt Disclosure [Abstract]
Contractual interest	$ 4,492	$ 4,492	$ 13,476	$ 13,614
Amortization of discount	1,186	1,140	3,533	3,525
Total interest expense recognized	$ 5,678	$ 5,632	$ 17,009	$ 17,139