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Debt (Details)
1 Months Ended 2 Months Ended 12 Months Ended
Mar. 31, 2013
USD ($)
Feb. 26, 2016
USD ($)
Dec. 31, 2015
USD ($)
Period
$ / shares
shares
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Debt Instrument [Line Items]          
Long-term debt     $ 1,223,000,000 $ 1,296,400,000  
Stated interest rate (in hundredths)     2.00%    
Repurchased senior notes, fair value     $ 73,957,000 21,767,000 $ 17,235,000
Pretax charge resulting from repurchase of notes     507,000 837,000 0
Net proceeds from convertible senior notes     0 0 $ 484,625,000
Subsequent Event          
Debt Instrument [Line Items]          
FHLBC advances   $ 155,000,000      
FHLBC fixed interest rate   1.91%      
Senior Notes          
Debt Instrument [Line Items]          
Long-term debt     $ 0 61,900,000  
Minimum percentage of consolidated shareholders' equity that determines designated subsidiary status     15.00%    
Senior Notes | Senior Notes Due 2015          
Debt Instrument [Line Items]          
Long-term debt       $ 61,900,000  
Stated interest rate (in hundredths)     5.375% 5.375%  
Maturity date     Nov. 01, 2015 Nov. 01, 2015  
Convertible Senior Notes Due 2017          
Debt Instrument [Line Items]          
Long-term debt     $ 333,500,000 $ 345,000,000  
Stated interest rate (in hundredths)     5.00% 5.00%  
Maturity date     May 01, 2017 May 01, 2017  
Repurchased senior notes, fair value     $ 12,000,000    
Pretax charge resulting from repurchase of notes     $ (500,000)    
Conversion rate (in shares per $1,000 note) | shares     74.4186    
Principal amount of notes used in determining conversion rate     $ 1,000    
Initial conversion price (in dollars per share) | $ / shares     $ 13.44    
Extinguishment of debt, amount     $ 11,500,000    
Convertible Senior Notes Due 2017 | Subsequent Event          
Debt Instrument [Line Items]          
Extinguishment of debt, amount   $ 127,700,000      
Convertible Senior Notes - due April 2020          
Debt Instrument [Line Items]          
Long-term debt     $ 500,000,000 $ 500,000,000  
Stated interest rate (in hundredths)     2.00% 2.00%  
Maturity date     Apr. 01, 2020 Apr. 01, 2020  
Conversion rate (in shares per $1,000 note) | shares     143.8332    
Principal amount of notes used in determining conversion rate     $ 1,000    
Initial conversion price (in dollars per share) | $ / shares     $ 6.95    
Net proceeds from convertible senior notes $ 484,600,000        
Minimum number of trading days     20 days    
Maximum number of trading days     30 days    
Percentage of conversion price (in hundredths)     130.00%    
130% of conversion price (in dollars per share) | $ / shares     $ 9.03    
Redemption price, percentage (in hundredths)     100.00%    
Convertible Junior Subordinated Debentures Due 2063          
Debt Instrument [Line Items]          
Long-term debt     $ 389,500,000 $ 389,500,000  
Stated interest rate (in hundredths)     9.00% 9.00%  
Maturity date     Apr. 01, 2063 Apr. 01, 2063  
Conversion rate (in shares per $1,000 note) | shares     74.0741    
Principal amount of notes used in determining conversion rate     $ 1,000    
Initial conversion price (in dollars per share) | $ / shares     $ 13.50    
Minimum number of trading days     20 days    
Maximum number of trading days     30 days    
Percentage of conversion price (in hundredths)     130.00%    
130% of conversion price (in dollars per share) | $ / shares     $ 17.55    
Redemption price, percentage (in hundredths)     100.00%    
Period preceding election to convert     5 days    
Minimum number of consecutive interest periods for which interest payments may be deferred | Period     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     tenth    
Net proceeds cap (in hundredths)     2.00%    
Maximum number of shares of common stock issuable under the Alternative Payment Mechanism (in shares) | shares     10,000,000    
Maximum percentage of aggregate principal amount of the debentures (in hundredths)     25.00%    
Convertible Junior Debentures at 9% per annum, Due 2063          
Debt Instrument [Line Items]          
Stated interest rate (in hundredths)     9.00%    
Convertible Junior Debentures at 9% per annum, Due 2063 | Subsequent Event          
Debt Instrument [Line Items]          
Extinguishment of debt, amount   $ 132,700,000