Fair Value Measurements - Liabilities Not Measured at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
5% Convertible Senior Notes due 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Stated interest rate (in hundredths)	5.00%
2% Convertible Senior Notes due 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Stated interest rate (in hundredths)	2.00%
Senior Notes | 5.75% Senior Notes due 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Stated interest rate (in hundredths)	5.75%
9% Convertible Junior Subordinated Debentures due 2063
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Stated interest rate (in hundredths)	9.00%
Carrying Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
FHLB Advance	$ 155,000	$ 155,000
5% Notes	0	144,789
2% Notes	0	204,672
5.75% Notes	418,560	417,406
9% Debentures	256,872	256,872
Total financial liabilities	830,432	1,178,739
Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
FHLB Advance	152,124	151,905
5% Notes	0	147,679
2% Notes	0	308,605
5.75% Notes	465,473	445,987
9% Debentures	353,507	323,040
Total financial liabilities	$ 971,104	$ 1,377,216