Commitments and contingencies - Credit facilities (Details) - USD ($) $ in Thousands	Sep. 20, 2019	Jun. 30, 2020	May 15, 2020	Dec. 31, 2019	Nov. 30, 2017
Line of Credit Facility [Line Items]
Revolving credit agreement borrowings		$ 472,361		$ 484,287
Lloyds Bank PLC | Letter of credit facility
Line of Credit Facility [Line Items]
Credit facility, maximum borrowing capacity			$ 100,000
Allowable capacity increase			$ 50,000
Restricted assets		48,700		51,000
Lloyds Bank and BMO Capital Markets | Letter of credit facility
Line of Credit Facility [Line Items]
Credit facility, maximum borrowing capacity	$ 100,000
Restricted assets		25,200		0
Credit Facility Term	364 days
Revolving credit agreement borrowings		25,200		19,300
Bank Of America, N.A. | Secured facility
Line of Credit Facility [Line Items]
Credit facility, maximum borrowing capacity					$ 800,000
Bank Of America, N.A. | Borrowings
Line of Credit Facility [Line Items]
Revolving credit agreement borrowings		335,600		484,300
Bank Of America, N.A. | Standby letters of credit
Line of Credit Facility [Line Items]
Credit facility, maximum borrowing capacity					$ 400,000
Revolving credit agreement borrowings		$ 52,500		$ 52,500