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Investments - Summary of Private Debt and Equity (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Net Investment Income [Line Items]
Total other invested assets	$ 45,621		$ 45,621		$ 61,210
Investment income	533	$ 1,384	1,494	$ 5,917
Renewable energy LLCs
Net Investment Income [Line Items]
Total other invested assets	30,520		30,520		31,219
Investment income	(526)	1,602	320	2,510
Renewable energy notes receivable
Net Investment Income [Line Items]
Total other invested assets	0		0		8,750
Investment income	$ 547	328	814	984
Repayment received			$ 8,800
Rate of interest	15.00%		15.00%
Limited partnerships
Net Investment Income [Line Items]
Total other invested assets	$ 10,601		$ 10,601		16,741
Investment income	426	(631)	102	2,166
Bank holding companies
Net Investment Income [Line Items]
Total other invested assets	4,500		4,500		$ 4,500
Investment income	$ 86	$ 85	258	257
Interest income			$ 257	257
Bank Holding Company
Net Investment Income [Line Items]
Rate of interest	7.60%		7.60%
Corporate and Other \| Limited partnerships
Net Investment Income [Line Items]
Carrying values of limited partnerships held	$ 3,500		$ 3,500
Investment income (loss)			(100)	182
Corporate and Other \| Investment in LLC
Net Investment Income [Line Items]
Cash distributions from LLCs			1,000	687
Corporate and Other \| Bank Holding Company
Net Investment Income [Line Items]
Investment in private subordinated notes	4,500		4,500
Excess and Surplus Lines \| Limited partnerships
Net Investment Income [Line Items]
Carrying values of limited partnerships held	7,100		7,100
Investment income (loss)			2	$ 2,000
Outstanding commitments to invest	$ 3,900		$ 3,900
Minimum \| Corporate and Other \| Investment in LLC
Net Investment Income [Line Items]
Ownership percentage	2.60%		2.60%
Maximum \| Corporate and Other \| Investment in LLC
Net Investment Income [Line Items]
Ownership percentage	32.60%		32.60%