Share-based compensation - Restricted Stock Units (Details) - Unvested restricted stock units $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2021 USD ($) $ / shares shares	Sep. 30, 2021 USD ($) $ / shares shares
Units
Outstanding, beginning of period (in shares) | shares		1,293,107
Granted (in shares) | shares	413,234	2,725,991
Vested (in shares) | shares		(507,776)
Forfeited (in shares) | shares		(84,132)
Outstanding, end of period (in shares) | shares	3,427,190	3,427,190
Per Unit Fair Value
Outstanding, beginning of period (in dollars per share) | $ / shares		$ 26.34
Granted (in dollars per share) | $ / shares		34.86
Vested (in dollars per share) | $ / shares		30.32
Forfeited (in dollars per share) | $ / shares		27.64
Outstanding, end of period (in dollars per share) | $ / shares	$ 32.49	$ 32.49
Vesting period	1 year	4 years
Unrecognized compensation expense | $	$ 102.2	$ 102.2
Weighted average period over which unrecognized compensation expense would be recognized	3 years 3 months 18 days
Tranche one
Per Unit Fair Value
Vesting period		1 year
Percentage of RSUs that vest	50.00%	100.00%