Fair Value Measurements - Recurring Measurements of Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Aug. 31, 2016	Dec. 31, 2015
5% Convertible Senior Notes due 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Stated interest rate (in hundredths)	5.00%
2% Convertible Senior Notes due 2020
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Stated interest rate (in hundredths)	2.00%
Senior Notes | 5.75% Senior Notes due 2023
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Stated interest rate (in hundredths)	5.75%	5.75%
9% Convertible Junior Subordinated Debentures due 2063
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Stated interest rate (in hundredths)	9.00%
Carrying Value
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
FHLB Advance	$ 155,000
5% Notes	144,789		$ 331,546
2% Notes	204,672		490,755
5.75% Notes	417,406
9% Debentures	256,872		389,522
Total financial liabilities	1,178,739		1,211,823
Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
FHLB Advance	151,905
5% Notes	147,679		345,616
2% Notes	308,605		701,955
5.75% Notes	445,987
9% Debentures	323,040		455,067
Total financial liabilities	$ 1,377,216		$ 1,502,638