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Debt (Details) (USD $)
Share data in Millions, except Per Share data, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2013
Period
Mar. 31, 2012
Dec. 31, 2012
Debt Instrument [Line Items]      
Debt instrument description of event of default we fail to meet any of the covenants of the 5% Notes and such failure continues for 60 days after we receive notice from holders of 25% or more of the 5% Notes    
Net proceeds from convertible senior notes $ 484,670,000 $ 0  
Amortized value of notes recorded as a liability on the consolidated balance sheet 389,522,000   379,609,000
Holding company cash and investments 671,000,000    
Unrealized gain loss on holding company investments 1,300,000    
Modified duration of holding company investments 2 years    
Par Value [Member]
     
Liabilities [Abstract]      
Senior Notes 100,118,000   100,118,000
Convertible Senior Notes due 2017 345,000,000   345,000,000
Convertible Senior Notes due 2020 500,000,000    
Convertible Junior Subordinated Debentures 389,522,000   389,522,000
Total Debt 1,334,640,000   834,640,000
Fair Value [Member]
     
Liabilities [Abstract]      
Senior Notes 98,266,000   79,594,000
Convertible Senior Notes due 2017 341,550,000   242,880,000
Convertible Senior Notes due 2020 511,565,000    
Convertible Junior Subordinated Debentures 390,009,000   173,096,000
Total Debt 1,341,390,000   495,570,000
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
     
Liabilities [Abstract]      
Senior Notes 98,266,000   79,594,000
Convertible Senior Notes due 2017 341,550,000   242,880,000
Convertible Senior Notes due 2020 511,565,000    
Convertible Junior Subordinated Debentures 0   0
Total Debt 951,381,000   322,474,000
Significant Other Observable Inputs (Level 2) [Member]
     
Liabilities [Abstract]      
Senior Notes 0   0
Convertible Senior Notes due 2017 0   0
Convertible Senior Notes due 2020 0    
Convertible Junior Subordinated Debentures 390,009,000   173,096,000
Total Debt 390,009,000   173,096,000
Significant Unobservable Inputs (Level 3) [Member]
     
Liabilities [Abstract]      
Senior Notes 0   0
Convertible Senior Notes due 2017 0   0
Convertible Senior Notes due 2020 0    
Convertible Junior Subordinated Debentures 0   0
Total Debt 0   0
Senior Notes [Member]
     
Debt Instrument [Line Items]      
Minimum percentage of consolidated shareholders' equity that determines designated subsidiary status (in hundredths) 15.00%    
Restriction on acceleration of other debt 40,000,000    
Default period 30 days    
Minimum percentage of aggregate notes held to grant acceleration rights to note holders (in hundredths) 25.00%    
Interest payments made 0 0  
Senior Notes [Member] | Senior Notes Due 2015 [Member]
     
Debt Instrument [Line Items]      
Outstanding principal amount 100,100,000   100,100,000
Stated interest rate (in hundredths) 5.375%   5.375%
Maturity date November 2015   November 2015
Annual rate (in hundredths) 5.375%   5.375%
Convertible Senior Notes - due April 2020 [Member]
     
Debt Instrument [Line Items]      
Outstanding principal amount 500,000,000    
Stated interest rate (in hundredths) 2.00%    
Maturity date April 1, 2020    
Restriction on acceleration of other debt 40,000,000    
Default period 30 days    
Minimum percentage of aggregate notes held to grant acceleration rights to note holders (in hundredths) 25.00%    
Restriction on final judgments 40,000,000    
Conversion rate (in shares per $1,000 note) 143.8332    
Principal amount of notes used in determining conversion rate 1,000    
Initial conversion price (in dollars per share) $ 6.95    
Net proceeds from convertible senior notes 484,700,000    
Redemption price, percentage (in hundredths) 100.00%    
Percentage of conversion price (in hundredths) 130.00%    
Minimum number of trading days 20 days    
Maximum number of trading days 30 days    
Annual rate (in hundredths) 2.00%    
Convertible Senior Notes Due 2017 [Member]
     
Debt Instrument [Line Items]      
Outstanding principal amount 345,000,000   345,000,000
Stated interest rate (in hundredths) 5.00%   5.00%
Maturity date May 1, 2017    
Restriction on acceleration of other debt 40,000,000    
Default period 30 days    
Minimum percentage of aggregate notes held to grant acceleration rights to note holders (in hundredths) 25.00%    
Interest payments made 0 0  
Restriction on final judgments 40,000,000    
Conversion rate (in shares per $1,000 note) 74.4186    
Principal amount of notes used in determining conversion rate 1,000    
Initial conversion price (in dollars per share) $ 13.44    
Annual rate (in hundredths) 5.00%   5.00%
Convertible Junior Subordinated Debentures Due 2063 [Member]
     
Debt Instrument [Line Items]      
Outstanding principal amount 389,500,000   389,500,000
Stated interest rate (in hundredths) 9.00%   9.00%
Maturity date April 1, 2063    
Minimum percentage of aggregate notes held to grant acceleration rights to note holders (in hundredths) 25.00%    
Interest payments made 0 0  
Conversion rate (in shares per $1,000 note) 74.0741    
Principal amount of notes used in determining conversion rate 1,000    
Initial conversion price (in dollars per share) $ 13.50    
Redemption price, percentage (in hundredths) 100.00%    
Percentage of conversion price (in hundredths) 130.00%    
Minimum number of trading days 20 days    
Maximum number of trading days 30 days    
Effective interest rate (in hundredths) 19.00%   19.00%
Amortized value of notes recorded as a liability on the consolidated balance sheet     379,600,000
Minimum number of consecutive interest periods for which interest payments may be deferred 1    
Maximum period for which interest payments may be deferred without giving rise to an event of default 10 years    
Annual rate (in hundredths) 9.00%   9.00%
Interest payment deferred 18,300,000    
Interest payment due on the debentures $ 17,500,000    
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    
Period in which reasonable commercial efforts must begin, minimum fifth anniversary of the interest payment date    
Net proceeds cap (in hundredths) 2.00%    
Maximum number of shares of common stock issuable under the Alternative Payment Mechanism (in shares) 10    
Maximum percentage of aggregate principal amount of the debentures (in hundredths) 25.00%    
Period preceding election to convert 5 days