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Investments - Summary of Private Debt and Equity (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Net Investment Income [Line Items]
Net realized gain (loss) on changes in fair value of impaired bank loans	$ (896)	$ (42)	$ (893)	$ 135
Total other invested assets	76,813		76,813		$ 70,208
Investment income	2,036	2,359	3,857	8,421
Renewable energy LLCs
Net Investment Income [Line Items]
Total other invested assets	32,204		32,204		32,063
Investment income	530	1,521	1,741	7,115
Renewable energy bridge financing notes
Net Investment Income [Line Items]
Total other invested assets	8,750		8,750		7,278
Investment income	328	49	$ 625	49
Rate of interest			15.00%
Interest income	328	49	$ 625	49
Limited partnerships
Net Investment Income [Line Items]
Total other invested assets	31,359		31,359		26,367
Investment income	1,092	703	1,319	1,085
Bank Holding Companies [Member]
Net Investment Income [Line Items]
Total other invested assets	4,500		4,500		$ 4,500
Investment income	86	86	172	172
Interest income	86	86	$ 172	172
Bank Holding Company
Net Investment Income [Line Items]
Rate of interest			7.60%
Interest income			$ 3
Investment in private subordinated notes			4,500
Corporate and other \| Limited partnerships
Net Investment Income [Line Items]
Carrying values of limited partnerships held	3,200		3,200
Investment income (loss)	333	152	208	220
Corporate and other \| Investment in LLC
Net Investment Income [Line Items]
Cash distributions from LLCs			2,100	3,600
Excess and Surplus Lines \| Limited partnerships
Net Investment Income [Line Items]
Carrying values of limited partnerships held	28,100		28,100
Investment income (loss)	759	$ 551	1,100	$ 865
Commitment to investment in limited partnership	$ 625		$ 625
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	32.80%		32.80%