Share-based compensation - Schedule of restricted shares with service and performance condition (Details) - Restricted shares with service and performance condition - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Number of non-vested restricted shares (in shares)
Outstanding, beginning of period (in shares)	1,890,529	2,001,048
Granted (in shares)	749,322	862,176
Forfeited (in shares)	(372,476)	(823,977)
Vested (in shares)	(504,440)	(148,718)
Outstanding, end of period (in shares)	1,762,935	1,890,529
Number of non-vested restricted shares probable of vesting
Outstanding, beginning of period (in shares)	1,314,036	1,185,220
Granted (in shares)	499,542	574,784
Forfeited (in shares)	39,214	(290,552)
Vested (in shares)	$ (504,440)	$ (148,718)
Change in estimated restricted shares considered probably of vesting (in shares)	(1,215)	(6,698)
Outstanding, end of period (in shares)	1,268,709	1,314,036
Weighted average grant data fair value (in dollars per share)
Outstanding (in dollars per share)	$ 12.43	$ 12.80
Granted (in dollars per share)	9.46	10.95
Forfeited (in dollars per share)	12.12	11.70
Vested (in dollars per share)	12.16	11.40
Change in estimated restricted shares considered probable of vesting (in dollars per share)	12.61	12.92
Outstanding (in dollars per share)	$ 11.36	$ 12.43