Supplemental equity information - Summary of Stock-Option Awards (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Number of Options (In thousands)
Outstanding beginning (in shares)	6,068,000
Granted (in shares)	0
Outstanding ending (in shares)	6,068,000	6,068,000
Exercisable at September 30, 2020 (in shares)	3,936,000
Weighted-Average Grant Date Fair Value
Outstanding beginning (in dollars per shares)	$ 1.78
Granted (in dollars per shares)	0
Outstanding ending (in dollars per shares)	1.78	$ 1.78
Exercisable at September 30, 2020 (in dollars per share)	1.78
Weighted-Average Exercise Price
Outstanding beginning (in dollars per shares)	14.70
Granted (in dollars per shares)	0
Outstanding ending (in dollars per shares)	13.97	$ 14.70
Exercisable at September 30, 2020 (in dollars per shares)	$ 13.15
Weighted-Average Remaining Contractual Term (Years)
Weighted-average remaining contractual term (Years)	7 years 4 months 24 days	8 years 2 months 12 days
Weighted-average remaining contractual term (Years), exercisable	6 years 6 months
Aggregate Intrinsic Value ($000)
Aggregate intrinsic value	$ 0	$ 0
Exercisable at September 30, 2020	$ 0