Fair Value Measurements - Assets and Liabilities Not Measured at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Carrying Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets	$ 3,100	$ 3,100
FHLB Advance	155,000	155,000
5.75% Senior Notes	420,002	419,713
9% Convertible Junior Subordinated Debentures	256,872	256,872
Total financial liabilities	831,874	831,585
Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets	3,100	3,100
FHLB Advance	153,133	150,551
5.75% Senior Notes	449,387	425,791
9% Convertible Junior Subordinated Debentures	332,580	338,069
Total financial liabilities	$ 935,100	$ 914,411
Senior Notes | 5.75% Senior Notes Due 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Stated interest rate	5.75%
9% Convertible Junior Subordinated Debentures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Stated interest rate	9.00%