Derivatives - Fair values and notional amounts of derivatives (Details) - Not designated as hedging instrument - USD ($) $ in Thousands		Jun. 30, 2020	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Asset Derivatives		$ 6	$ 1,815
Liability Derivatives		6,935	257
Net Derivatives		(6,929)	1,558
Notional Amount	[1]	175,762	222,557
Underwriting derivatives
Derivatives, Fair Value [Line Items]
Asset Derivatives		6	148
Liability Derivatives		0	0
Net Derivatives		6	148
Notional Amount	[1]	52,970	59,879
Total return swaps
Derivatives, Fair Value [Line Items]
Asset Derivatives		0	1,667
Liability Derivatives		6,935	257
Net Derivatives		(6,935)	1,410
Notional Amount	[1]	$ 122,792	$ 162,678

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