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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Debt Disclosure [Abstract]
Contractual interest	$ 4,785	$ 4,867	$ 14,519	$ 14,616
Amortization of discount	1,268	1,243	3,827	3,716
Total interest expense recognized	$ 6,053	$ 6,110	$ 18,346	$ 18,332