Share-based compensation - Unvested Share-based Awards Activity (Details) - shares	3 Months Ended	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2020	Jun. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Granted (in shares)			1,374,944
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Unvested, beginning of the year (in shares)	1,374,945	1,374,945
Granted (in shares)	0	0	34,615
Vested (in shares)	0	0
Forfeited (in shares)	0	0
Unvested, end of the year (in shares)	1,374,945	1,374,945
Performance share units
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Granted (in shares)			401,311
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Granted (in shares)			14,335
RSUs
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested, beginning of the year (in shares)	1,320,837	1,353,852
Granted (in shares)	0	0	1,411,714
Vested (in shares)	0	0
Forfeited (in shares)	46,085	79,100
Unvested, end of the year (in shares)	1,274,752	1,274,752
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Granted (in shares)			46,052
IPO incentive awards | Performance share units
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested, beginning of the year (in shares)	543,196	555,163
Granted (in shares)	0	0
Vested (in shares)	0	0
Forfeited (in shares)	0	11,967
Unvested, end of the year (in shares)	543,196	543,196
Long term incentive plan 2019 | Performance share units
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested, beginning of the year (in shares)	382,327	391,136
Granted (in shares)	0	0
Vested (in shares)	0	0
Forfeited (in shares)	4,824	13,633
Unvested, end of the year (in shares)	377,503	377,503
2018 Long Term Incentive Plan (LTIP)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested, beginning of the year (in shares)	856,099	870,471
Granted (in shares)	0	0
Vested (in shares)	0	0
Forfeited (in shares)	9,942	24,314
Unvested, end of the year (in shares)	846,157	846,157