Investments in Fixed Maturity Securities (The Amortized Cost and Fair Value of Fixed Maturity Securities) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 884,831
Amortized Cost, Due after one year through five years	3,953,003
Amortized Cost, Due after five years through ten years	3,600,155
Amortized Cost, Due after ten years	2,485,594
Amortized Cost, Mortgage-backed securities	1,299,681
Total investment in fixed maturity securities, Amortized Cost	12,223,264	11,291,631
Fair Value, Due in one year or less	895,004
Fair Value, Due after one year through five years	4,119,946
Fair Value, Due after five years through ten years	3,790,402
Fair Value, Due after ten years	2,598,686
Fair Value, Mortgage-backed securities	1,321,768
Total investments in fixed maturity securities, Fair Value	$ 12,725,806	$ 11,631,744