Investments - Summary of Private Debt and Equity (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	May 31, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Net Investment Income [Line Items]
Net realized gain (loss) on changes in fair value of impaired bank loans		$ 729	$ 663	$ 594	$ 53
Total other invested assets		66,205		66,205		$ 55,419
Investment income		2,316	4,779	10,737	6,467
Maturities and repayments				0	6,500
Renewable energy LLCs
Net Investment Income [Line Items]
Total other invested assets		31,264		31,264		27,067
Investment income		1,516	2,745	8,631	1,976
Renewable energy bridge financing notes
Net Investment Income [Line Items]
Total other invested assets		5,688		5,688		0
Investment income		238	0	$ 287	450
Rate of interest				15.00%
Commitment to invest		$ 3,100		$ 3,100
Commitment fee percentage		2.50%		2.50%
Interest income		$ 238		$ 287
Maturities and repayments	$ 6,500
Limited partnerships
Net Investment Income [Line Items]
Total other invested assets		24,753		24,753		23,852
Investment income		477	1,948	1,562	3,784
Bank holding companies
Net Investment Income [Line Items]
Total other invested assets		4,500		4,500		$ 4,500
Investment income		85	$ 86	257	257
Interest income				$ 257	257
Bank Holding Company
Net Investment Income [Line Items]
Investment income					266
Rate of interest				7.60%
Interest income				$ 2	3
Investment in private subordinated notes				4,500
Corporate and other | Limited partnerships
Net Investment Income [Line Items]
Carrying values of limited partnerships held		2,900		2,900
Investment income (loss)				246	178
Corporate and other | Investment in LLC
Net Investment Income [Line Items]
Cash distributions from LLCs				4,400	2,400
Excess and Surplus Lines | Limited partnerships
Net Investment Income [Line Items]
Carrying values of limited partnerships held		21,900		21,900
Investment income (loss)				1,300	$ 3,600
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%