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Debt (Details)
1 Months Ended 3 Months Ended 12 Months Ended
Aug. 31, 2020
USD ($)
Dec. 31, 2020
USD ($)
Period
$ / shares
shares
Sep. 30, 2020
USD ($)
Jun. 30, 2020
USD ($)
Mar. 31, 2020
USD ($)
Dec. 31, 2020
USD ($)
Period
$ / shares
shares
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Debt Instrument [Line Items]                
Early repayment of debt           $ 179,735,000 $ 0 $ 0
Loss on debt extinguishment   $ 0 $ 26,736,000 $ 0 $ 0 26,736,000 0 0
FHLBC advances   $ 155,000,000.0       $ 155,000,000.0    
FHLBC fixed interest rate   1.91%       1.91%    
Collateral pledged to principal balance of FHLB advance (as a percent)           102.00%    
Interest paid           $ 54,300,000 $ 50,800,000 $ 51,300,000
Senior Notes | 5.75% Senior Notes due 2023                
Debt Instrument [Line Items]                
Stated interest rate (in hundredths)   5.75%       5.75%    
Debt instrument, face amount   $ 242,300,000       $ 242,300,000    
Extinguishment of debt           182,700,000    
Early repayment of debt           197,800,000    
Loss on debt extinguishment           $ 16,500,000    
Redemption price, percentage (in hundredths)           100.00%    
Ownership percentage threshold for declaration of due and payable           25.00%    
Senior Notes | 5.25% Senior Notes due 2028                
Debt Instrument [Line Items]                
Stated interest rate (in hundredths) 5.25% 5.25%       5.25%    
Debt instrument, face amount $ 650,000,000 $ 650,000,000       $ 650,000,000    
Proceeds from issuance of debt 640,300,000              
Debt issuance costs $ 2,000,000.0              
Ownership percentage threshold for declaration of due and payable           25.00%    
Senior Notes | 5.25% Senior Notes due 2028 | Debt Instrument, Redemption, Period One                
Debt Instrument [Line Items]                
Redemption price, percentage (in hundredths)           102.625%    
Senior Notes | 5.25% Senior Notes due 2028 | Debt Instrument, Redemption, Period Two                
Debt Instrument [Line Items]                
Redemption price, percentage (in hundredths)           102.625%    
Senior Notes | 5.25% Senior Notes due 2028 | Debt Instrument, Redemption, Period Two | US Treasury (UST) Interest Rate                
Debt Instrument [Line Items]                
Basis spread on variable rate           0.50%    
Senior Notes | 5.25% Senior Notes due 2028 | Debt Instrument, Redemption, Period Three                
Debt Instrument [Line Items]                
Redemption price, percentage (in hundredths)           102.625%    
Senior Notes | 5.25% Senior Notes due 2028 | Debt Instrument, Redemption, Period Four                
Debt Instrument [Line Items]                
Redemption price, percentage (in hundredths)           101.313%    
Senior Notes | 5.25% Senior Notes due 2028 | Debt Instrument, Redemption, Period Five                
Debt Instrument [Line Items]                
Redemption price, percentage (in hundredths)           100.00%    
Convertible Junior Subordinated Debentures, at 9% per annum, Due 2063                
Debt Instrument [Line Items]                
Stated interest rate (in hundredths)   9.00%       9.00%    
Extinguishment of debt           $ 48,100,000    
Loss on debt extinguishment           10,200,000    
Repayments of long-term debt           61,600,000    
Reacquisition of convertible junior subordinated debentures-equity component           $ 2,700,000    
Reduction in potentially dilutive shares due to debt extinguishment | shares           3,600,000    
Redemption price, percentage (in hundredths)           100.00%    
Conversion price (in dollars per share) | $ / shares   $ 13.23       $ 13.23    
Conversion rate (in shares per $1,000 note) | shares           75.5932    
Principal amount of notes used in determining conversion rate   $ 1,000       $ 1,000    
Period preceding election to convert           5 days    
Closing sale price of our common stock for consideration of redemption (in dollars per share) | $ / shares   $ 17.20       $ 17.20    
Minimum number of trading days           20 days    
Maximum number of trading days           30 days    
Minimum number of consecutive interest periods for which interest payments may be deferred | Period   1       1    
Maximum period for which interest payments may be deferred without giving rise to an event of default           10 years    
Period in which reasonable commercial efforts must begin, maximum           1 day    
Anniversary payment release of the start of the interest deferral to the Alternative Payment Mechanism in lieu of the final maturity of the debentures           10 years    
Net proceeds cap (in hundredths)   2.00%       2.00%    
Maximum number of shares of common stock issuable under the Alternative Payment Mechanism (in shares) | shares   10,000,000       10,000,000    
Maximum percentage of aggregate principal amount of the debentures (in hundredths)   25.00%       25.00%