Derivatives - Summary of realized and unrealized gains and losses on derivatives (Details) - Not designated as hedging instrument - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Other underwriting income (loss) | Underwriting derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Realized and unrealized gains and losses on derivative instruments	$ 133	$ 592
Realized and unrealized gains (losses) on investments | Options
Derivative Instruments, Gain (Loss) [Line Items]
Realized and unrealized gains and losses on derivative instruments	1,081	801
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	$ (23,882)	$ 1,582