Commitments and contingencies - Leases (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Leases
Operating lease expense	$ 2.9	$ 3.2	$ 6.0	$ 5.8
Operating lease right-of-use assets	28.8		28.8		$ 27.4
Lease liability	$ 30.5		$ 30.5		$ 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:OtherLiabilities		us-gaap:OtherLiabilities		us-gaap:OtherLiabilities
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilities		us-gaap:OtherLiabilities		us-gaap:OtherLiabilities
Leased offices
Leases
Weighted average lease term (years)	7 years		7 years
Leased equipment
Leases
Weighted average lease term (years)	3 years		3 years