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Investments (Details) (USD $)
 3 Months Ended 12 Months Ended
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2010
Sep. 30, 2010
Jun. 30, 2010
Mar. 31, 2010
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Schedule of Available-for-sale Securities [Line Items]                      
Amortized Cost $ 5,703,560,000       $ 7,369,857,000       $ 5,703,560,000 $ 7,369,857,000  
Gross Unrealized Gains 134,673,000       161,988,000       134,673,000 161,988,000  
Gross Unrealized Losses (14,586,000) [1]       (73,563,000) [1]       (14,586,000) [1] (73,563,000) [1]  
Fair Value 5,823,647,000       7,458,282,000       5,823,647,000 7,458,282,000  
Percentage of auction rate securities with contractual maturities greater than 10 years (in hundredths) 100.00%               100.00%    
Contractual maturity of auction rate securities (in years) 10Y               10Y    
Net impairment losses recognized in earnings                 715,000 9,644,000 40,940,000
Maturities, Amortized Cost [Abstract]                      
Total at end of period 5,700,894,000       7,366,808,000       5,700,894,000 7,366,808,000  
Maturities, Fair Value [Abstract]                      
Total at end of period 5,823,647,000       7,458,282,000       5,823,647,000 7,458,282,000  
Continuous Unrealized Loss Portion, Fair Value [Abstract]                      
Less than 12 months 1,077,710,000       2,345,030,000       1,077,710,000 2,345,030,000  
12 months or greater 166,172,000       388,259,000       166,172,000 388,259,000  
Total investment portfolio 1,243,882,000       2,733,289,000       1,243,882,000 2,733,289,000  
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]                      
Less than 12 months 7,873,000       50,234,000       7,873,000 50,234,000  
12 months or greater 6,713,000       23,329,000       6,713,000 23,329,000  
Total investment portfolio 14,586,000       73,563,000       14,586,000 73,563,000  
OTTI losses in other comprehensive income                 1,800,000    
Percentage of foreign investments held in government and semi-government securities (in hundredths) 94.00%               94.00%    
Percentage of foreign investments held in corporate securities (in hundredths) 6.00%               6.00%    
Schedule of Investment Income, Reported Amounts, by Category [Line Items]                      
Investment income                 204,053,000 250,452,000 307,891,000
Investment expenses                 (2,783,000) (3,199,000) (3,213,000)
Net investment income 40,339,000 48,898,000 55,490,000 56,543,000 57,061,000 58,465,000 62,868,000 68,859,000 201,270,000 247,253,000 304,678,000
Gain (Loss) on Investments [Line Items]                      
Net realized investment gains (losses) and OTTI on investments                 142,715,000 92,937,000 51,934,000
Change in net unrealized appreciation (depreciation)                 31,662,000 (71,308,000) 235,402,000
Net unrealized holding gains (losses) arising during the period, net of tax, included in accumulated other comprehensive income                 68,822,000 (7,534,000) 143,378,000
Less: net gains (losses) reclassified out of accumulated other comprehensive income into earnings for the period                 47,765,000 61,540,000 (9,216,000)
Change in unrealized investment gains (losses), net of tax                 21,057,000 (69,074,000) 152,594,000
Tax expense (benefit) related to the changes in net unrealized (depreciation) appreciation                 10,600,000 1,000,000 82,800,000
Gross realized gains, gross realized losses and impairment losses [Abstract]                      
Gross realized gains                 158,659,000 119,325,000 112,148,000
Gross realized losses                 (15,229,000) (16,278,000) (19,274,000)
Impairment losses                 (715,000) (9,644,000) (40,940,000)
Net realized gains (losses) on securities                 142,715,000 93,403,000 51,934,000
Loss from joint ventures                 0 (466,000) 0
Total net realized gains (losses)                 142,715,000 92,937,000 51,934,000
Investments on deposit with various states 22,300,000       21,800,000       22,300,000 21,800,000  
Joint Ventures [Member]
                      
Gain (Loss) on Investments [Line Items]                      
Net realized investment gains (losses) and OTTI on investments                 0 (466,000) 0
Gross realized gains, gross realized losses and impairment losses [Abstract]                      
Total net realized gains (losses)                 0 (466,000) 0
Available-for-sale Securities [Member] | Fixed Maturities [Member]
                      
Gain (Loss) on Investments [Line Items]                      
Net realized investment gains (losses) and OTTI on investments                 142,284,000 93,017,000 51,109,000
Change in net unrealized appreciation (depreciation)                 31,576,000 (71,304,000) 237,521,000
Gross realized gains, gross realized losses and impairment losses [Abstract]                      
Total net realized gains (losses)                 142,284,000 93,017,000 51,109,000
Available-for-sale Securities [Member] | Equity Securities [Member]
                      
Gain (Loss) on Investments [Line Items]                      
Net realized investment gains (losses) and OTTI on investments                 330,000 151,000 116,000
Change in net unrealized appreciation (depreciation)                 86,000 (4,000) 144,000
Gross realized gains, gross realized losses and impairment losses [Abstract]                      
Total net realized gains (losses)                 330,000 151,000 116,000
Available-for-sale Securities [Member] | Other [Member]
                      
Gain (Loss) on Investments [Line Items]                      
Net realized investment gains (losses) and OTTI on investments                 101,000 235,000 709,000
Change in net unrealized appreciation (depreciation)                 0 0 (2,263,000)
Gross realized gains, gross realized losses and impairment losses [Abstract]                      
Total net realized gains (losses)                 101,000 235,000 709,000
Fixed Maturities [Member]
                      
Schedule of Investment Income, Reported Amounts, by Category [Line Items]                      
Investment income                 202,301,000 236,734,000 291,304,000
Equity Securities [Member]
                      
Schedule of Investment Income, Reported Amounts, by Category [Line Items]                      
Investment income                 330,000 315,000 819,000
Cash Equivalents [Member]
                      
Schedule of Investment Income, Reported Amounts, by Category [Line Items]                      
Investment income                 496,000 1,526,000 3,056,000
Interest on Sherman Note [Member]
                      
Schedule of Investment Income, Reported Amounts, by Category [Line Items]                      
Investment income                 0 10,796,000 11,323,000
Other [Member]
                      
Schedule of Investment Income, Reported Amounts, by Category [Line Items]                      
Investment income                 926,000 1,081,000 1,389,000
Available-for-sale Securities [Member]
                      
Credit Losses Recognized in Earnings [Roll Forward]                      
Beginning balance       0       1,021,000 0 1,021,000  
Addition for the amount related to the credit loss for which an OTTI was not previously recognized                 0 0  
Additional increases to the amount related to the credit loss for which an OTTI was previously recognized                 0 0  
Reductions for securities sold during the period (realized)                 0 (1,021,000)  
Ending balance 0       0       0 0  
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member]
                      
Schedule of Available-for-sale Securities [Line Items]                      
Amortized Cost 592,108,000       1,092,890,000       592,108,000 1,092,890,000  
Gross Unrealized Gains 4,965,000       16,718,000       4,965,000 16,718,000  
Gross Unrealized Losses (36,000) [1]       (6,822,000) [1]       (36,000) [1] (6,822,000) [1]  
Fair Value 597,037,000       1,102,786,000       597,037,000 1,102,786,000  
Maturities, Fair Value [Abstract]                      
Total at end of period 597,037,000       1,102,786,000       597,037,000 1,102,786,000  
Continuous Unrealized Loss Portion, Fair Value [Abstract]                      
Less than 12 months 78,546,000       258,235,000       78,546,000 258,235,000  
12 months or greater 0       0       0 0  
Total investment portfolio 78,546,000       258,235,000       78,546,000 258,235,000  
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]                      
Less than 12 months 36,000       6,822,000       36,000 6,822,000  
12 months or greater 0       0       0 0  
Total investment portfolio 36,000       6,822,000       36,000 6,822,000  
Obligations of U.S. States and Political Subdivisions [Member]
                      
Schedule of Available-for-sale Securities [Line Items]                      
Amortized Cost 2,255,192,000       3,549,355,000       2,255,192,000 3,549,355,000  
Gross Unrealized Gains 74,918,000       85,085,000       74,918,000 85,085,000  
Gross Unrealized Losses (6,639,000) [1]       (54,374,000) [1]       (6,639,000) [1] (54,374,000) [1]  
Fair Value 2,323,471,000       3,580,066,000       2,323,471,000 3,580,066,000  
Maturities, Fair Value [Abstract]                      
Total at end of period 2,323,471,000       3,580,066,000       2,323,471,000 3,580,066,000  
Continuous Unrealized Loss Portion, Fair Value [Abstract]                      
Less than 12 months 188,879,000       1,160,877,000       188,879,000 1,160,877,000  
12 months or greater 137,965,000       359,629,000       137,965,000 359,629,000  
Total investment portfolio 326,844,000       1,520,506,000       326,844,000 1,520,506,000  
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]                      
Less than 12 months 837,000       32,415,000       837,000 32,415,000  
12 months or greater 5,802,000       21,959,000       5,802,000 21,959,000  
Total investment portfolio 6,639,000       54,374,000       6,639,000 54,374,000  
Corporate Debt Securities [Member]
                      
Schedule of Available-for-sale Securities [Line Items]                      
Amortized Cost 2,007,720,000       2,521,275,000       2,007,720,000 2,521,275,000  
Gross Unrealized Gains 32,750,000       54,975,000       32,750,000 54,975,000  
Gross Unrealized Losses (7,619,000) [1]       (11,291,000) [1]       (7,619,000) [1] (11,291,000) [1]  
Fair Value 2,032,851,000       2,564,959,000       2,032,851,000 2,564,959,000  
Maturities, Fair Value [Abstract]                      
Total at end of period 2,032,851,000       2,564,959,000       2,032,851,000 2,564,959,000  
Continuous Unrealized Loss Portion, Fair Value [Abstract]                      
Less than 12 months 689,396,000       817,471,000       689,396,000 817,471,000  
12 months or greater 28,174,000       28,630,000       28,174,000 28,630,000  
Total investment portfolio 717,570,000       846,101,000       717,570,000 846,101,000  
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]                      
Less than 12 months 6,709,000       9,921,000       6,709,000 9,921,000  
12 months or greater 910,000       1,370,000       910,000 1,370,000  
Total investment portfolio 7,619,000       11,291,000       7,619,000 11,291,000  
Residential Mortgage-Backed Securities [Member]
                      
Schedule of Available-for-sale Securities [Line Items]                      
Amortized Cost 441,589,000       53,845,000       441,589,000 53,845,000  
Gross Unrealized Gains 4,113,000       3,255,000       4,113,000 3,255,000  
Gross Unrealized Losses (285,000) [1]       0 [1]       (285,000) [1] 0 [1]  
Fair Value 445,417,000       57,100,000       445,417,000 57,100,000  
Maturities, Amortized Cost [Abstract]                      
Debt securities without single maturity date 441,589,000               441,589,000    
Maturities, Fair Value [Abstract]                      
Debt securities without single maturity date 445,417,000               445,417,000    
Total at end of period 445,417,000       57,100,000       445,417,000 57,100,000  
Continuous Unrealized Loss Portion, Fair Value [Abstract]                      
Less than 12 months 120,405,000       0       120,405,000 0  
12 months or greater 0       0       0 0  
Total investment portfolio 120,405,000       0       120,405,000 0  
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]                      
Less than 12 months 285,000       0       285,000 0  
12 months or greater 0       0       0 0  
Total investment portfolio 285,000       0       285,000 0  
Debt Securities Issued by Foreign Sovereign Governments [Member]
                      
Schedule of Available-for-sale Securities [Line Items]                      
Amortized Cost 146,755,000       149,443,000       146,755,000 149,443,000  
Gross Unrealized Gains 10,441,000       1,915,000       10,441,000 1,915,000  
Gross Unrealized Losses (6,000) [1]       (1,031,000) [1]       (6,000) [1] (1,031,000) [1]  
Fair Value 157,190,000       150,327,000       157,190,000 150,327,000  
Maturities, Fair Value [Abstract]                      
Total at end of period 157,190,000       150,327,000       157,190,000 150,327,000  
Continuous Unrealized Loss Portion, Fair Value [Abstract]                      
Less than 12 months 484,000       105,724,000       484,000 105,724,000  
12 months or greater 0       0       0 0  
Total investment portfolio 484,000       105,724,000       484,000 105,724,000  
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]                      
Less than 12 months 6,000       1,031,000       6,000 1,031,000  
12 months or greater 0       0       0 0  
Total investment portfolio 6,000       1,031,000       6,000 1,031,000  
Total Debt Securities [Member]
                      
Schedule of Available-for-sale Securities [Line Items]                      
Amortized Cost 5,700,894,000       7,366,808,000       5,700,894,000 7,366,808,000  
Gross Unrealized Gains 134,591,000       161,948,000       134,591,000 161,948,000  
Gross Unrealized Losses (14,585,000) [1]       (73,518,000) [1]       (14,585,000) [1] (73,518,000) [1]  
Fair Value 5,820,900,000       7,455,238,000       5,820,900,000 7,455,238,000  
Maturities, Amortized Cost [Abstract]                      
Due in one year or less 1,238,386,000               1,238,386,000    
Due after one year through five years 1,860,588,000               1,860,588,000    
Due after five years through ten years 897,967,000               897,967,000    
Due after ten years 828,573,000               828,573,000    
Total debt securities with single maturity date 4,825,514,000               4,825,514,000    
Total at end of period 5,700,894,000               5,700,894,000    
Maturities, Fair Value [Abstract]                      
Due in one year or less 1,240,917,000               1,240,917,000    
Due after one year through five years 1,900,400,000               1,900,400,000    
Due after five years through ten years 942,561,000               942,561,000    
Due after ten years 856,774,000               856,774,000    
Total debt securities with single maturity date 4,940,652,000               4,940,652,000    
Total at end of period 5,820,900,000       7,455,238,000       5,820,900,000 7,455,238,000  
Equity Securities [Member]
                      
Schedule of Available-for-sale Securities [Line Items]                      
Amortized Cost 2,666,000       3,049,000       2,666,000 3,049,000  
Gross Unrealized Gains 82,000       40,000       82,000 40,000  
Gross Unrealized Losses (1,000) [1]       (45,000) [1]       (1,000) [1] (45,000) [1]  
Fair Value 2,747,000       3,044,000       2,747,000 3,044,000  
Maturities, Fair Value [Abstract]                      
Total at end of period 2,747,000       3,044,000       2,747,000 3,044,000  
Continuous Unrealized Loss Portion, Fair Value [Abstract]                      
Less than 12 months 0       2,723,000       0 2,723,000  
12 months or greater 33,000       0       33,000 0  
Total investment portfolio 33,000       2,723,000       33,000 2,723,000  
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]                      
Less than 12 months 0       45,000       0 45,000  
12 months or greater 1,000       0       1,000 0  
Total investment portfolio 1,000       45,000       1,000 45,000  
Auction Rate Securities [Member]
                      
Maturities, Amortized Cost [Abstract]                      
Debt securities without single maturity date 176,261,000 [2]               176,261,000 [2]    
Maturities, Fair Value [Abstract]                      
Debt securities without single maturity date 169,897,000 [2]               169,897,000 [2]    
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]                      
Fair value of ARS backed by student loans 170,000,000       358,000,000       170,000,000 358,000,000  
Auction process, interest rate reset interval, low (in days) 7               7    
Auction process, interest rate reset interval, mid (in days) 28               28    
Auction process, interest rate reset interval, high (in days) 35               35    
Percentage of ARS held guaranteed by U.S. Department of Education (in hundredths) 97.00%               97.00%    
Percentage of ARS portfolio rated AAA/Aaa by one or more of the major rating agencies (in hundredths) 83.00%               83.00%    
Number of investments in ARS portfolio 19               19    
The par value of ARS sold or called                 361,000,000    
Percentage of par value received in sale or call of ARS (in hundredths) 97.00%               97.00%    
Commercial Mortgage-Backed Securities [Member]
                      
Schedule of Available-for-sale Securities [Line Items]                      
Amortized Cost 257,530,000               257,530,000    
Gross Unrealized Gains 7,404,000               7,404,000    
Gross Unrealized Losses 0 [1]               0 [1]    
Fair Value 264,934,000               264,934,000    
Maturities, Amortized Cost [Abstract]                      
Debt securities without single maturity date 257,530,000               257,530,000    
Maturities, Fair Value [Abstract]                      
Debt securities without single maturity date 264,934,000               264,934,000    
Total at end of period $ 264,934,000               $ 264,934,000    
[1] There were no other-than-temporary impairment losses recorded in other comprehensive income at December 31, 2011 and 2010.
[2] At December 31, 2011, 100% of auction rate securities had a contractual maturity greater than 10 years.