Derivatives - Fair values and notional amounts of derivatives (Details) - Not designated as hedging instrument - USD ($) $ in Thousands		Mar. 31, 2020	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Asset Derivatives		$ 0	$ 1,815
Liability Derivatives		24,040	257
Net Derivatives		(24,040)	1,558
Notional Amount	[1]	169,668	222,557
Other underwriting derivatives
Derivatives, Fair Value [Line Items]
Asset Derivatives		0	148
Liability Derivatives		314	0
Net Derivatives		(314)	148
Notional Amount	[1]	56,923	59,879
Total return swaps
Derivatives, Fair Value [Line Items]
Asset Derivatives		0	1,667
Liability Derivatives		23,726	257
Net Derivatives		(23,726)	1,410
Notional Amount	[1]	$ 112,745	$ 162,678

[1] The notional amount represents the absolute value of all outstanding contracts.