Fair Value Measurements - Assets and Liabilities Not Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Aug. 31, 2020	Dec. 31, 2019
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
9% Convertible Junior Subordinated Debentures	208,814		256,872
Total financial liabilities	1,242,652		832,739
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	161,436		156,422
9% Convertible Junior Subordinated Debentures	268,397		346,289
Total financial liabilities	$ 1,360,376		974,538
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%	5.75%
Senior Notes | 5.75% Senior Notes Due 2023 | Carrying Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	$ 240,433		420,867
Senior Notes | 5.75% Senior Notes Due 2023 | Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	$ 257,065		471,827
Senior Notes | 5.25% Senior Notes Due 2028
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Stated interest rate	5.25%	5.25%
Senior Notes | 5.25% Senior Notes Due 2028 | Carrying Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	$ 638,405		0
Senior Notes | 5.25% Senior Notes Due 2028 | Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	$ 673,478		$ 0
9% Convertible Junior Subordinated Debentures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Stated interest rate	9.00%