Investments in Fixed Maturity Securities (The Amortized Cost and Fair Value of Fixed Maturity Securities) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Investments in Fixed Maturity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 671,995
Amortized Cost, Due after one year through five years	3,486,501
Amortized Cost, Due after five years through ten years	2,527,716
Amortized Cost, Due after ten years	2,834,568
Amortized Cost, Mortgage-backed securities	1,697,174
Total investment in fixed maturity securities, Amortized Cost	11,217,954	10,754,156
Fair Value, Due in one year or less	691,130
Fair Value, Due after one year through five years	3,669,945
Fair Value, Due after five years through ten years	2,801,283
Fair Value, Due after ten years	3,052,271
Fair Value, Mortgage-backed securities	1,753,747
Total investments in fixed maturity securities, Fair Value	$ 11,968,376	$ 11,330,910