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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Debt Disclosure [Abstract]
Contractual interest	$ 4,492	$ 4,561	$ 8,984	$ 9,122
Amortization of discount	1,185	1,176	2,347	2,385
Total interest expense recognized	$ 5,677	$ 5,737	$ 11,331	$ 11,507