Investment information - Net investment income (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020		Mar. 31, 2019
Net Investment Income [Line Items]
Allowance for expected credit losses on available for sale securities recognized in period		$ 600
Net interest income		27,803		$ 30,434
Net unrealized gains (losses)		(285,456)		32,438
Net realized gains (losses)		(5,046)		1,282
Net investment income (loss)		(262,699)		58,354
Investment management fees - related parties
Net Investment Income [Line Items]
Net interest income		(4,352)		(4,409)
Net investment income (loss)		(4,352)		(4,409)
Borrowing and miscellaneous other investment expenses
Net Investment Income [Line Items]
Net interest income		(5,669)		(8,298)
Net investment income (loss)		(5,669)		(8,298)
Investment performance fees - related parties
Net Investment Income [Line Items]
Net investment income (loss)				(5,800)
Term loans
Net Investment Income [Line Items]
Net interest income		21,018		22,410
Net unrealized gains (losses)		(155,233)		9,366
Net realized gains (losses)		(8,145)		64
Net investment income (loss)		(142,360)		31,840
Fixed maturities - Fair value option
Net Investment Income [Line Items]
Net interest income		9,068		16,043
Net unrealized gains (losses)		(89,675)		19,975
Net realized gains (losses)		(1,244)		(749)
Net investment income (loss)		(81,851)		35,269
Fixed maturities - Available for sale
Net Investment Income [Line Items]
Net interest income		4,660		3,422
Net unrealized gains (losses)		0		0
Net realized gains (losses)		2,180		395
Net investment income (loss)		6,840	[1]	3,817	[2]
Short-term investments
Net Investment Income [Line Items]
Net interest income		1,850		524
Net unrealized gains (losses)		(3,440)		(448)
Net realized gains (losses)		87		0
Net investment income (loss)		(1,503)		76
Equities
Net Investment Income [Line Items]
Net interest income		0		120
Net unrealized gains (losses)		76		2,158
Net realized gains (losses)		1,041		0
Net investment income (loss)	[3]	1,117		2,278
Equities, fair value through net income
Net Investment Income [Line Items]
Net interest income		377		622
Net unrealized gains (losses)		(12,275)		(1,989)
Net realized gains (losses)		(212)		1,572
Net investment income (loss)	[3]	(12,110)		205
Other investments
Net Investment Income [Line Items]
Net interest income		851		0
Net unrealized gains (losses)		220		1,794
Net realized gains (losses)		0		0
Net investment income (loss)		1,071		1,794
Other
Net Investment Income [Line Items]
Net interest income		0		0
Net unrealized gains (losses)		(25,129)		1,582
Net realized gains (losses)		1,247		0
Net investment income (loss)	[4]	$ (23,882)		$ 1,582

[1] Net realized gains (losses) from the fixed maturities available for sale portfolio consists of realized gains and realized losses of $3.3 million and $1.1 million, respectively. Realized losses include an allowance for expected credit losses on available for sale securities of $0.6 million for the three months ended March 31, 2020.

[2] Net realized gains (losses) from the fixed maturities available for sale portfolio consists of realized gains and realized losses of $498.0 thousand and $103.0 thousand, respectively.

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

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