Commitments and contingencies - Leases (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Leases
Operating lease expense	$ 3.0	$ 3.4	$ 9.0	$ 9.3
Operating lease right-of-use assets	27.1		27.1		$ 27.4
Lease liability	$ 29.0		$ 29.0		$ 29.3
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssets		us-gaap:OtherAssets		us-gaap:OtherAssets
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent		us-gaap:OtherLiabilitiesCurrent		us-gaap:OtherLiabilitiesCurrent
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent		us-gaap:OtherLiabilitiesNoncurrent		us-gaap:OtherLiabilitiesNoncurrent
Leased offices
Leases
Weighted average lease term (years)	7 years		7 years
Leased equipment
Leases
Weighted average lease term (years)	3 years		3 years