Investments in Fixed Maturity Securities (The Amortized Cost and Fair Value of Fixed Maturity Securities) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Marketable Securities, Fixed Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Amortized Cost, Due in one year or less	$ 900,989
Amortized Cost, Due after one year through five years	3,555,283
Amortized Cost, Due after five years through ten years	2,686,863
Amortized Cost, Due after ten years	2,490,039
Amortized Cost, Mortgage-backed securities	1,568,860
Total investment in fixed maturity securities, Amortized Cost	11,202,034	11,217,954
Fair Value, Due in one year or less	925,233
Fair Value, Due after one year through five years	3,697,764
Fair Value, Due after five years through ten years	2,853,103
Fair Value, Due after ten years	2,582,858
Fair Value, Mortgage-backed securities	1,593,424
Total investments in fixed maturity securities, Fair Value	$ 11,652,382	$ 11,968,376