Equity-based compensation - Summary of Unvested Holdings Unit Activity (Detail) - Unvested Holdings Units $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares shares
Holdings Units
Units Outstanding at beginning of period (in shares) | shares	270,221
Units granted (in shares) | shares	0
Units forfeited (in shares) | shares	(8,990)
Units vested (in shares) | shares	(247,746)
Units Outstanding at end of period (in shares) | shares	13,485
Weighted average grant date fair value
Weighted average grant date fair value, Outstanding at beginning of period (in usd per share) | $ / shares	$ 1.52
Weighted average grant date fair value, Units granted (in usd per share) | $ / shares	0.00
Weighted average grant date fair value, Units forfeited (in usd per share) | $ / shares	1.52
Weighted average grant date fair value, Units vested (usd per share) | $ / shares	1.52
Weighted average grant date fair value, Outstanding at end of period (in usd per share) | $ / shares	$ 1.52
Weighted average remaining contractual term, Outstanding at end of period (in years)	10 months 24 days
Aggregate intrinsic value, Outstanding at end of period | $	$ 723