Share-based compensation - Unvested Share-based Awards Activity (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
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	1,374,944
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)		7,059		408,370
Shares forfeited (in shares)		43,230		57,565
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)	1,274,752		1,353,852
Granted (in shares)	0	16,471	0	1,428,185
Vested (in shares)	0		0
Shares forfeited (in shares)	1,060	25,936	80,160	71,988
Unvested, end of the year (in shares)	1,273,692		1,273,692