Investment information - Narrative (Details) $ in Thousands	3 Months Ended	6 Months Ended		12 Months Ended
	Jun. 30, 2020 USD ($) positions	Jun. 30, 2020 USD ($) positions	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) positions	Mar. 31, 2020 USD ($)
Investments, Debt and Equity Securities [Abstract]
Number of positions in an unrealized loss position | positions	72	72		48
Total number of positions | positions	158	158		146
Percentage of positions in unrealized loss				10.00%
Allowance for expected credit losses on available for sale securities recognized in period	$ (200)	$ 400
Allowance for expected credit losses on available for sale securities	400	400			$ 600
Restricted Assets [Line Items]
Transfer from investments		33,236	$ 0
Restricted assets	2,200,000	2,200,000		$ 2,100,000
Deposits with US regulatory authorities
Restricted Assets [Line Items]
Restricted assets	$ 6,700	$ 6,700		6,400
Term loans
Restricted Assets [Line Items]
Transfer from investments				16,900
Short-term investments
Restricted Assets [Line Items]
Transfer from investments				6,500
Equities, fair value through net income
Restricted Assets [Line Items]
Transfer from investments				28,700
Transfer to investments				23,000
Other investments, fair value option
Restricted Assets [Line Items]
Transfer to investments				$ 28,700