Investments in Fixed Maturity Securities (The Amortized Cost and Fair Value of Fixed Maturity Securities) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Investments in Fixed Maturity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 865,253
Amortized Cost, Due after one year through five years	3,287,781
Amortized Cost, Due after five years through ten years	2,494,584
Amortized Cost, Due after ten years	2,912,517
Amortized Cost, Mortgage-backed securities	1,615,149
Total investment in fixed maturity securities, Amortized Cost	11,175,284	10,754,156
Fair Value, Due in one year or less	882,883
Fair Value, Due after one year through five years	3,463,096
Fair Value, Due after five years through ten years	2,719,280
Fair Value, Due after ten years	3,112,330
Fair Value, Mortgage-backed securities	1,671,261
Total investments in fixed maturity securities, Fair Value	$ 11,848,850	$ 11,330,910