Investments in Fixed Maturity Securities (The Amortized Cost and Fair Value of Fixed Maturity Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 711,642
Amortized Cost, Due after one year through five years	4,056,450
Amortized Cost, Due after five years through ten years	3,653,308
Amortized Cost, Due after ten years	2,476,413
Amortized Cost, Mortgage-backed securities	1,210,495
Total investment in fixed maturity securities, Amortized Cost	12,108,308	$ 12,223,264
Fair Value, Due in one year or less	717,075
Fair Value, Due after one year through five years	4,185,436
Fair Value, Due after five years through ten years	3,795,614
Fair Value, Due after ten years	2,543,518
Fair Value, Mortgage-backed securities	1,221,204
Total investments in fixed maturity securities, Fair Value	$ 12,462,847	$ 12,725,806