Fair Value Measurements - Summary of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Available-for-sale:
Fixed maturity securities	$ 1,043,251	$ 1,016,098
Equity securities	85,957	82,522
Trading:
Bank loan participations held-for-investment, at amortized cost, net of allowance	257,426	238,214
Cash and cash equivalents	151,046	163,495	$ 94,224	$ 109,784
Short-term investments	26,235	36,804
Liabilities
Senior debt	98,300	98,300
Junior subordinated debt	104,055	104,055
Carrying Value
Available-for-sale:
Fixed maturity securities	1,043,251	1,016,098
Equity securities	85,957	82,522
Trading:
Fixed maturity securities	3,805	3,808
Bank loan participations held-for-investment, at amortized cost, net of allowance	257,426	238,214
Cash and cash equivalents	151,046	163,495
Short-term investments	26,235	36,804
Other invested assets – notes receivable	13,250	11,778
Liabilities
Senior debt	98,300	98,300
Junior subordinated debt	104,055	104,055
Fair Value
Available-for-sale:
Fixed maturity securities	1,043,251	1,016,098
Equity securities	85,957	82,522
Trading:
Fixed maturity securities	3,805	3,808
Bank loan participations held-for-investment, at amortized cost, net of allowance	257,840	236,532
Cash and cash equivalents	151,046	163,495
Short-term investments	26,235	36,804
Other invested assets – notes receivable	19,056	17,104
Liabilities
Senior debt	98,871	97,884
Junior subordinated debt	$ 118,752	$ 116,569