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Investments - Summary of Private Debt and Equity (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	May 31, 2017	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Net Investment Income [Line Items]
Total other invested assets		$ 65,481		$ 65,481		$ 55,419
Investment income		2,359	$ 522	8,421	$ 1,689
Interest income				1
Maturities and repayments				0	6,500
Renewable energy LLCs
Net Investment Income [Line Items]
Total other invested assets		30,572		30,572		27,067
Investment income		1,521	(1,451)	7,115	(769)
Renewable energy bridge financing notes
Net Investment Income [Line Items]
Total other invested assets		5,688		5,688		0
Investment income		49	207	$ 49	450
Rate of interest				15.00%
Commitment to invest		$ 3,100		$ 3,100
Commitment fee percentage		2.50%		2.50%
Interest income				$ 49
Maturities and repayments	$ 6,500
Limited partnerships
Net Investment Income [Line Items]
Total other invested assets		$ 24,721		24,721		23,852
Investment income		703	1,680	1,085	1,836
Bank holding companies
Net Investment Income [Line Items]
Total other invested assets		4,500		4,500		$ 4,500
Investment income		86	$ 86	$ 172	172
Bank Holding Company
Net Investment Income [Line Items]
Investment income					233
Rate of interest				7.60%
Interest income				$ 172	172
Investment in private subordinated notes				4,500
Corporate and other \| Limited partnerships
Net Investment Income [Line Items]
Carrying values of limited partnerships held		2,800		2,800
Investment income (loss)				220	235
Corporate and other \| Investment in LLC
Net Investment Income [Line Items]
Cash distributions from LLCs				3,600	1,500
Excess and Surplus Lines \| Limited partnerships
Net Investment Income [Line Items]
Carrying values of limited partnerships held		21,900		21,900
Investment income (loss)				865	$ 1,600
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%