Share-based compensation - Unvested Share-based Awards Activity (Details) - shares	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Unvested, beginning of the year (in shares)	1,374,945
Granted (in shares)	0	1,374,945
Vested (in shares)	0
Forfeited (in shares)	0
Unvested, end of the year (in shares)	1,374,945
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)	1,353,852
Granted (in shares)	0	1,411,714
Vested (in shares)	0
Forfeited (in shares)	33,015
Unvested, end of the year (in shares)	1,320,837
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)	555,163
Granted (in shares)	0
Vested (in shares)	0
Forfeited (in shares)	11,967
Unvested, end of the year (in shares)	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)	391,136
Granted (in shares)	0	401,311
Vested (in shares)	0
Forfeited (in shares)	8,809
Unvested, end of the year (in shares)	382,327
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)	870,471
Granted (in shares)	0
Vested (in shares)	0
Forfeited (in shares)	14,372
Unvested, end of the year (in shares)	856,099