Investment information - Narrative (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($) positions	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) positions
Investments, Debt and Equity Securities [Abstract]
Number of positions in an unrealized loss position | positions	103		48
Total - gross unrealized losses on fixed maturities	$ 47,314,000		$ 1,990,000
Total number of positions | positions	140		146
Percentage of positions in unrealized loss			10.00%
Allowance for expected credit losses on available for sale securities recognized in period	$ 600,000
Fixed maturities, AFS, realized gain	3,300,000	$ 498,000
Fixed maturities, AFS, realized loss	1,100,000	103,000
Restricted Assets [Line Items]
Allowance for expected credit losses on available for sale securities	600,000
Transfer from investments	13,467,000	$ 0
Restricted assets	2,100,000,000		$ 2,100,000,000
Deposits with US regulatory authorities
Restricted Assets [Line Items]
Restricted assets	5,100,000		6,400,000
Corporate bonds
Investments, Debt and Equity Securities [Abstract]
Total - gross unrealized losses on fixed maturities	4,969,000		41,000
Restricted Assets [Line Items]
Unrealized losses on investments with credit losses	5,700,000
Equities, fair value through net income
Restricted Assets [Line Items]
Transfer from investments	28,700,000
Transfer to investments			23,000,000
Other investments, fair value option
Restricted Assets [Line Items]
Transfer to investments	28,700,000
Term loans
Restricted Assets [Line Items]
Transfer from investments	13,500,000		16,900,000
Short-term investments
Restricted Assets [Line Items]
Transfer from investments			$ 6,500,000
Transfer to investments	$ 13,500,000