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Investments - Summary of Private Debt and Equity (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Net Investment Income [Line Items]
Total other invested assets	$ 47,463			$ 47,463		$ 61,210
Investment income	444		$ 1,129	961	$ 4,533
Renewable energy LLCs
Net Investment Income [Line Items]
Total other invested assets	31,318			31,318		31,219
Investment income	12		(13)	846	908
Renewable energy notes receivable
Net Investment Income [Line Items]
Total other invested assets	2,680			2,680		8,750
Investment income	$ 101		328	$ 267	656
Repayment received		$ 6,100
Rate of interest	15.00%			15.00%
Limited partnerships
Net Investment Income [Line Items]
Total other invested assets	$ 8,965			$ 8,965		16,741
Investment income	245		728	(324)	2,797
Bank holding companies
Net Investment Income [Line Items]
Total other invested assets	4,500			4,500		$ 4,500
Investment income	$ 86		$ 86	172	172
Interest income				$ 172	172
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	$ 3,200			$ 3,200
Investment income (loss)				(201)	480
Corporate and Other \| Investment in LLC
Net Investment Income [Line Items]
Cash distributions from LLCs				747	253
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	$ 5,800			5,800
Investment income (loss)				$ 123	$ 2,300
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%