Fair Value Measurements - Summary of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Available-for-sale:
Fixed maturity securities	$ 1,016,818	$ 941,077
Equity securities	78,705	76,401
Trading:
Bank loan participations held-for-investment, at amortized cost, net of allowance	245,741	203,526
Cash and cash equivalents	106,500	109,784	$ 90,873	$ 106,406
Short-term investments	42,331	50,844
Liabilities
Senior debt	88,300	88,300
Junior subordinated debt	104,055	104,055
Carrying Value
Available-for-sale:
Fixed maturity securities	1,016,818	941,077
Equity securities	78,705	76,401
Trading:
Fixed maturity securities	3,812	5,063
Bank loan participations held-for-investment, at amortized cost, net of allowance	245,741	203,526
Cash and cash equivalents	106,500	109,784
Short-term investments	42,331	50,844
Other invested assets – notes receivable	10,188	4,500
Liabilities
Senior debt	88,300	88,300
Junior subordinated debt	104,055	104,055
Fair Value
Available-for-sale:
Fixed maturity securities	1,016,818	941,077
Equity securities	78,705	76,401
Trading:
Fixed maturity securities	3,812	5,063
Bank loan participations held-for-investment, at amortized cost, net of allowance	242,164	203,123
Cash and cash equivalents	106,500	109,784
Short-term investments	42,331	50,844
Other invested assets – notes receivable	12,033	6,008
Liabilities
Senior debt	85,706	85,404
Junior subordinated debt	$ 109,384	$ 99,397