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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt Disclosure [Abstract]
Contractual interest	$ 4,492	$ 4,785	$ 13,614	$ 14,519
Amortization of discount	1,140	1,268	3,525	3,827
Total interest expense recognized	$ 5,632	$ 6,053	$ 17,139	$ 18,346