Investments in Fixed Maturity Securities (The Amortized Cost and Fair Value of Fixed Maturity Securities) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Investments in Fixed Maturity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 731,638
Amortized Cost, Due after one year through five years	3,057,341
Amortized Cost, Due after five years through ten years	2,646,432
Amortized Cost, Due after ten years	2,761,176
Amortized Cost, Mortgage-backed securities	1,557,569
Total investment in fixed maturity securities, Amortized Cost	10,754,156	10,853,193
Fair Value, Due in one year or less	742,334
Fair Value, Due after one year through five years	3,202,556
Fair Value, Due after five years through ten years	2,864,395
Fair Value, Due after ten years	2,895,669
Fair Value, Mortgage-backed securities	1,625,956
Total investments in fixed maturity securities, Fair Value	$ 11,330,910	$ 11,215,317