Fair Value (Financial Assets Measured At Fair Value On A Recurring Basis) (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Cash equivalents	$ 3,937,536	$ 1,606,592
Total debt securities	9,560,580	8,372,439
Securities lending invested collateral	5,486	49,636
Total invested assets	13,503,602	10,028,667
U.S. Treasury And Agency Obligations [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Debt Securities [Member] | U.S. Treasury And Other U.S. Government Corporations And Agencies [Member]
Total debt securities	0	0
Mortgage-Backed Securities [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Debt Securities [Member] | U.S. Treasury And Other U.S. Government Corporations And Agencies [Member]
Total debt securities	0	0
Tax-Exempt Municipal Securities [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Debt Securities [Member]
Total debt securities	0	0
Residential Mortgage-Backed Securities [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Debt Securities [Member]
Total debt securities	0	0
Commercial Mortgage-Backed Securities [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Debt Securities [Member]
Total debt securities	0	0
Asset-Backed Securities [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Debt Securities [Member]
Total debt securities	0	0
Corporate Debt Securities [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Debt Securities [Member]
Total debt securities	0	0
Redeemable Preferred Stock [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Debt Securities [Member]
Total debt securities	0	0
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Cash equivalents	3,937,536	1,606,592
Total debt securities	0	0
Securities lending invested collateral	5,486	24,639
Total invested assets	3,943,022	1,631,231
U.S. Treasury And Agency Obligations [Member] | Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | U.S. Treasury And Other U.S. Government Corporations And Agencies [Member]
Total debt securities	880,719	711,613
Mortgage-Backed Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | U.S. Treasury And Other U.S. Government Corporations And Agencies [Member]
Total debt securities	1,801,652	1,663,179
Tax-Exempt Municipal Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member]
Total debt securities	2,704,249	2,381,528
Residential Mortgage-Backed Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member]
Total debt securities	47,428	55,887
Commercial Mortgage-Backed Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member]
Total debt securities	387,911	321,031
Asset-Backed Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member]
Total debt securities	106,388	148,545
Corporate Debt Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member]
Total debt securities	3,579,918	3,025,097
Redeemable Preferred Stock [Member] | Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member]
Total debt securities	0	0
Significant Other Observable Inputs (Level 2) [Member]
Cash equivalents	0	0
Total debt securities	9,508,265	8,306,880
Securities lending invested collateral	0	24,997
Total invested assets	9,508,265	8,331,877
U.S. Treasury And Agency Obligations [Member] | Significant Unobservable Inputs (Level 3) [Member] | Debt Securities [Member] | U.S. Treasury And Other U.S. Government Corporations And Agencies [Member]
Total debt securities	0	0
Mortgage-Backed Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Debt Securities [Member] | U.S. Treasury And Other U.S. Government Corporations And Agencies [Member]
Total debt securities	0	0
Tax-Exempt Municipal Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Debt Securities [Member]
Total debt securities	21,440	51,806
Residential Mortgage-Backed Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Debt Securities [Member]
Total debt securities	0	0
Commercial Mortgage-Backed Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Debt Securities [Member]
Total debt securities	0	0
Asset-Backed Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Debt Securities [Member]
Total debt securities	1,065	1,206
Corporate Debt Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Debt Securities [Member]
Total debt securities	24,477	7,214
Redeemable Preferred Stock [Member] | Significant Unobservable Inputs (Level 3) [Member] | Debt Securities [Member]
Total debt securities	5,333	5,333
Significant Unobservable Inputs (Level 3) [Member]
Cash equivalents	0	0
Total debt securities	52,315	65,559
Securities lending invested collateral	0	0
Total invested assets	52,315	65,559
U.S. Treasury And Agency Obligations [Member] | Debt Securities [Member] | U.S. Treasury And Other U.S. Government Corporations And Agencies [Member]
Total debt securities	880,719	711,613
Mortgage-Backed Securities [Member] | Debt Securities [Member] | U.S. Treasury And Other U.S. Government Corporations And Agencies [Member]
Total debt securities	1,801,652	1,663,179
Tax-Exempt Municipal Securities [Member] | Debt Securities [Member]
Total debt securities	2,725,689	2,433,334
Residential Mortgage-Backed Securities [Member] | Debt Securities [Member]
Total debt securities	47,428	55,887
Commercial Mortgage-Backed Securities [Member] | Debt Securities [Member]
Total debt securities	387,911	321,031
Asset-Backed Securities [Member] | Debt Securities [Member]
Total debt securities	107,453	149,751
Corporate Debt Securities [Member] | Debt Securities [Member]
Total debt securities	3,604,395	3,032,311
Redeemable Preferred Stock [Member] | Debt Securities [Member]
Total debt securities	$ 5,333	$ 5,333