Investment information - Net investment income (Details) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2020		Jun. 30, 2019		Jun. 30, 2020		Jun. 30, 2019
Net Investment Income [Line Items]
Net interest income		$ 27,428		$ 26,415		$ 55,231		$ 56,849
Net unrealized gains (losses)		178,064		(1,725)		(107,392)		30,713
Net realized gains (losses)		(6,001)		789		(11,047)		2,071
Net investment income (loss)		199,491		23,787		(63,208)		82,141
Fixed maturities, AFS, realized gain		9,500		1,900		12,800		2,400
Fixed maturities, AFS, realized loss		600		100		1,700		200
Allowance for expected credit losses on available for sale securities recognized in period		(200)				400
Investment management fees - related parties
Net Investment Income [Line Items]
Net interest income		(4,262)		(4,570)		(8,614)		(8,979)
Net investment income (loss)		(4,262)		(4,570)		(8,614)		(8,979)
Borrowing and miscellaneous other investment expenses
Net Investment Income [Line Items]
Net interest income		(4,763)		(7,611)		(10,432)		(15,909)
Net investment income (loss)		(4,763)		(7,611)		(10,432)		(15,909)
Investment performance fees - related parties
Net Investment Income [Line Items]
Net investment income (loss)				(1,692)				(7,492)
Term loans
Net Investment Income [Line Items]
Net interest income		18,022		22,665		39,040		45,075
Net unrealized gains (losses)		90,951		(11,437)		(64,282)		(2,071)
Net realized gains (losses)		(6,142)		148		(14,287)		212
Net investment income (loss)		102,831		11,376		(39,529)		43,216
Fixed maturities - Fair value option
Net Investment Income [Line Items]
Net interest income		10,887		9,592		19,955		25,635
Net unrealized gains (losses)		45,781		8,995		(43,894)		28,970
Net realized gains (losses)		230		1,655		(1,014)		906
Net investment income (loss)		56,898		20,242		(24,953)		55,511
Fixed maturities - Available for sale
Net Investment Income [Line Items]
Net interest income		3,648		4,411		8,308		7,833
Net unrealized gains (losses)		0		0		0		0
Net realized gains (losses)		8,911		1,816		11,091		2,211
Net investment income (loss)		12,559	[1]	6,227	[2]	19,399	[3]	10,044	[4]
Short-term investments
Net Investment Income [Line Items]
Net interest income		2,881		1,055		4,731		1,579
Net unrealized gains (losses)		3,513		(48)		73		(496)
Net realized gains (losses)		38		25		125		25
Net investment income (loss)		6,432		1,032		4,929		1,108
Equities
Net Investment Income [Line Items]
Net interest income		0		80		0		200
Net unrealized gains (losses)		5,792		(1,096)		5,868		1,062
Net realized gains (losses)		(3,156)		0		(3,195)		0
Net investment income (loss)	[5]	2,636		(1,016)		2,673		1,262
Equities, fair value through net income
Net Investment Income [Line Items]
Net interest income		196		793		573		1,415
Net unrealized gains (losses)		11,781		3,687		(494)		1,698
Net realized gains (losses)		(6,085)		(3,599)		(5,217)		(2,027)
Net investment income (loss)	[5]	5,892		881		(5,138)		1,086
Other investments
Net Investment Income [Line Items]
Net interest income		819		0		1,670		0
Net unrealized gains (losses)		3,463		(2,213)		3,683		(419)
Net realized gains (losses)		0		(202)		0		(202)
Net investment income (loss)		4,282		(2,415)		5,353		(621)
Other
Net Investment Income [Line Items]
Net interest income		0		0		0		0
Net unrealized gains (losses)		16,783		387		(8,346)		1,969
Net realized gains (losses)		203		946		1,450		946
Net investment income (loss)	[6]	$ 16,986		$ 1,333		$ (6,896)		$ 2,915

[1] Net realized gains (losses) from the fixed maturities available for sale portfolio consists of realized gains and realized losses of $9.5 million and $0.6 million, respectively. Realized gains include a reversal of the allowance for expected credit losses on available for sale securities of $0.2 million for the three months ended June 30, 2020.

[2] Net realized gains (losses) from the fixed maturities available for sale portfolio consists of realized gains and realized losses of $1.9 million and $0.1 million, respectively.

[3] Net realized gains (losses) from the fixed maturities available for sale portfolio consists of realized gains and realized losses of $12.8 million and $1.7 million, respectively.

[4] Net realized gains (losses) from the fixed maturities available for sale portfolio consists of realized gains and realized losses of $2.4 million and $0.2 million, respectively.

[5] Net interest income includes dividends for securities held in long and short positions.

[6] Other includes unrealized gains and unrealized losses for total return swaps.