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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Debt Disclosure [Abstract]
Contractual interest	$ 4,561	$ 4,867	$ 9,122	$ 9,734
Amortization of discount	1,176	1,290	2,385	2,559
Total interest expense recognized	$ 5,737	$ 6,157	$ 11,507	$ 12,293