Investments in Fixed Maturity Securities (The Amortized Cost and Fair Value of Fixed Maturity Securities) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Investments in Fixed Maturity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 798,683
Amortized Cost, Due after one year through five years	3,482,465
Amortized Cost, Due after five years through ten years	2,536,987
Amortized Cost, Due after ten years	2,607,141
Amortized Cost, Mortgage-backed securities	1,528,316
Total investment in fixed maturity securities, Amortized Cost	10,953,592	11,217,954
Fair Value, Due in one year or less	826,292
Fair Value, Due after one year through five years	3,673,747
Fair Value, Due after five years through ten years	2,806,448
Fair Value, Due after ten years	2,800,766
Fair Value, Mortgage-backed securities	1,581,071
Total investments in fixed maturity securities, Fair Value	$ 11,688,324	$ 11,968,376