Schedule I - Summary Of Investments - Other Than Investments In Related Parties (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash, Cost	$ 15,165,054
Total investments and cash, Market Value	15,797,381
Total investments and cash, Amount Shown in Balance Sheet	15,797,381
U.S. Government And Government Agencies [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash, Cost	284,514
Total investments and cash, Market Value	300,634
Total investments and cash, Amount Shown in Balance Sheet	300,634
Obligations Of U.S. States And Political Subdivisions [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash, Cost	1,558,615
Total investments and cash, Market Value	1,660,905
Total investments and cash, Amount Shown in Balance Sheet	1,660,905
Foreign Government Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash, Cost	1,555,707
Total investments and cash, Market Value	1,668,218
Total investments and cash, Amount Shown in Balance Sheet	1,668,218
Foreign Corporate Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash, Cost	2,086,951
Total investments and cash, Market Value	2,146,631
Total investments and cash, Amount Shown in Balance Sheet	2,146,631
Public Utilities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash, Cost	323,921
Total investments and cash, Market Value	351,653
Total investments and cash, Amount Shown in Balance Sheet	351,653
All Other Corporate Bonds [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash, Cost	3,319,466
Total investments and cash, Market Value	3,471,858
Total investments and cash, Amount Shown in Balance Sheet	3,471,858
Commercial Mortgage-Backed Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash, Cost	310,387
Total investments and cash, Market Value	321,427
Total investments and cash, Amount Shown in Balance Sheet	321,427
Agency Residential [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash, Cost	2,198,937
Total investments and cash, Market Value	2,282,593
Total investments and cash, Amount Shown in Balance Sheet	2,282,593
Non-Agency Residential [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash, Cost	53,365
Total investments and cash, Market Value	53,089
Total investments and cash, Amount Shown in Balance Sheet	53,089
Redeemable Preferred Stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash, Cost	39,310
Total investments and cash, Market Value	36,516
Total investments and cash, Amount Shown in Balance Sheet	36,516
Total Fixed Maturities-Available For Sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash, Cost	11,731,173
Total investments and cash, Market Value	12,293,524
Total investments and cash, Amount Shown in Balance Sheet	12,293,524
Fixed Maturities - Available For Sale At Fair Value [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash, Cost	113,859	[1]
Total investments and cash, Market Value	113,606	[1]
Total investments and cash, Amount Shown in Balance Sheet	113,606	[1]
Equity Securities - Available For Sale At Market Value [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash, Cost	463,620
Total investments and cash, Market Value	448,930
Total investments and cash, Amount Shown in Balance Sheet	448,930
Equity Securities - Available For Sale At Fair Value [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash, Cost	1,164,187	[1]
Total investments and cash, Market Value	1,249,106	[1]
Total investments and cash, Amount Shown in Balance Sheet	1,249,106	[1]
Short-Term Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash, Cost	685,332
Total investments and cash, Market Value	685,332
Total investments and cash, Amount Shown in Balance Sheet	685,332
Other Invested Assets [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash, Cost	558,232
Total investments and cash, Market Value	558,232
Total investments and cash, Amount Shown in Balance Sheet	558,232
Cash [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash, Cost	448,651
Total investments and cash, Market Value	448,651
Total investments and cash, Amount Shown in Balance Sheet	$ 448,651

[1] Original cost does not reflect fair value adjustments, which have been realized through the statements of operations and comprehensive income (loss).