Investments in Fixed Maturity Securities (The Amortized Cost and Fair Value of Fixed Maturity Securities) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Marketable Securities, Fixed Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Amortized Cost, Due in one year or less	$ 824,784
Amortized Cost, Due after one year through five years	3,480,083
Amortized Cost, Due after five years through ten years	2,927,972
Amortized Cost, Due after ten years	2,486,202
Amortized Cost, Mortgage-backed securities	1,505,567
Total investment in fixed maturity securities, Amortized Cost	11,224,608	11,217,954
Fair Value, Due in one year or less	833,107
Fair Value, Due after one year through five years	3,625,395
Fair Value, Due after five years through ten years	3,091,396
Fair Value, Due after ten years	2,553,072
Fair Value, Mortgage-backed securities	1,525,880
Total investments in fixed maturity securities, Fair Value	$ 11,628,850	$ 11,968,376