Investments - Schedule of unobservable inputs (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Corporate bonds	$ 51,848	$ 218,280
Derivative liabilities (embedded)	180	92
Significant unobservable inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Corporate bonds	9,685	9,255
Derivative liabilities (embedded)	180	92
Private Equity Funds | Market Approach Valuation Technique | Significant unobservable inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investments, Fair Value Disclosure	$ 45,991	$ 47,608
Private Equity Funds | Market Approach Valuation Technique | Significant unobservable inputs (Level 3) | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Volatility	29.00%	40.00%
Fair Value Inputs, Discount Rate		5.00%
Fair Value Assumptions, Exit Term	9 months	4 months 24 days
Fair Value Assumptions, Multiples		2.0
Private Equity Funds | Market Approach Valuation Technique | Significant unobservable inputs (Level 3) | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Volatility	55.00%	60.00%
Fair Value Inputs, Discount Rate		25.00%
Fair Value Assumptions, Exit Term	3 years	2 years 9 months 18 days
Fair Value Assumptions, Multiples		3.8
Bank Debt [Member] | Discounted cash flow | Significant unobservable inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investments, Fair Value Disclosure	$ 10,284
Bank Debt [Member] | Discounted cash flow | Significant unobservable inputs (Level 3) | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Variable rate spread	1.024%
Debt Instrument, Term	1 year
Bank Debt [Member] | Discounted cash flow | Significant unobservable inputs (Level 3) | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Variable rate spread	1.144%
Debt Instrument, Term	4 years
Other debt securities | Discounted cash flow | Significant unobservable inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investments, Fair Value Disclosure	$ 7,268
Other debt securities | Discounted cash flow | Significant unobservable inputs (Level 3) | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	9.50%
Fair Value Inputs, Cap Rate	6.50%
Other debt securities | Discounted cash flow | Significant unobservable inputs (Level 3) | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Cap Rate	10.00%
Real estate | Discounted cash flow | Significant unobservable inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investments, Fair Value Disclosure	$ 2,755
Fair Value Inputs, Discount Rate	8.50%
Fair Value Inputs, Cap Rate	7.00%