Investments in Fixed Maturity Securities (The Amortized Cost and Fair Value of Fixed Maturity Securities) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Investments in Fixed Maturity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 877,420
Amortized Cost, Due after one year through five years	3,223,807
Amortized Cost, Due after five years through ten years	2,422,530
Amortized Cost, Due after ten years	2,973,511
Amortized Cost, Mortgage-backed securities	1,644,940
Total investment in fixed maturity securities, Amortized Cost	11,142,208	10,754,156
Fair Value, Due in one year or less	896,040
Fair Value, Due after one year through five years	3,416,358
Fair Value, Due after five years through ten years	2,688,914
Fair Value, Due after ten years	3,206,362
Fair Value, Mortgage-backed securities	1,709,048
Total investments in fixed maturity securities, Fair Value	$ 11,916,722	$ 11,330,910