Investments in Fixed Maturity Securities (The Amortized Cost and Fair Value of Fixed Maturity Securities) (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 935,354
Amortized Cost, Due after one year through five years	4,666,934
Amortized Cost, Due after five years through ten years	3,037,450
Amortized Cost, Due after ten years	3,414,764
Amortized Cost, Mortgage-backed securities	1,620,190
Total investment in fixed maturity securities, Amortized Cost	13,674,692
Fair Value, Due in one year or less	935,894
Fair Value, Due after one year through five years	4,669,502
Fair Value, Due after five years through ten years	3,045,868
Fair Value, Due after ten years	3,361,807
Fair Value, Mortgage-backed securities	1,606,549
Total investments in fixed maturity securities, Fair Value	$ 13,619,620