Derivatives - Summary of realized and unrealized gains and losses on derivatives (Details) - Not designated as hedging instrument - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Other underwriting income (loss) | Underwriting derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Realized and unrealized gains and losses on derivative instruments	$ 868	$ 673	$ 1,001	$ 1,265
Realized and unrealized gains (losses) on investments | Options
Derivative Instruments, Gain (Loss) [Line Items]
Realized and unrealized gains and losses on derivative instruments	0	(2)	1,081	799
Realized and unrealized gains (losses) on investments | Total return swaps
Derivative Instruments, Gain (Loss) [Line Items]
Realized and unrealized gains and losses on derivative instruments	$ 16,986	$ 1,333	$ (6,896)	$ 2,915