Investments in Fixed Maturity Securities (The Amortized Cost and Fair Value of Fixed Maturity Securities) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Investments in Fixed Maturity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 746,923
Amortized Cost, Due after one year through five years	3,193,375
Amortized Cost, Due after five years through ten years	2,586,044
Amortized Cost, Due after ten years	2,927,773
Amortized Cost, Mortgage-backed securities	1,519,778
Total investment in fixed maturity securities, Amortized Cost	10,973,893	10,754,156
Fair Value, Due in one year or less	757,854
Fair Value, Due after one year through five years	3,352,319
Fair Value, Due after five years through ten years	2,798,238
Fair Value, Due after ten years	3,089,175
Fair Value, Mortgage-backed securities	1,575,154
Total investments in fixed maturity securities, Fair Value	$ 11,572,740	$ 11,330,910