Investments in Fixed Maturity Securities (The Amortized Cost and Fair Value of Fixed Maturity Securities) (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 959,583
Amortized Cost, Due after one year through five years	5,010,862
Amortized Cost, Due after five years through ten years	3,391,154
Amortized Cost, Due after ten years	3,004,026
Amortized Cost, Mortgage-backed securities	1,611,022
Total investment in fixed maturity securities, Amortized Cost	13,976,647
Fair Value, Due in one year or less	917,059
Fair Value, Due after one year through five years	5,087,806
Fair Value, Due after five years through ten years	3,511,621
Fair Value, Due after ten years	3,043,510
Fair Value, Mortgage-backed securities	1,634,959
Total investments in fixed maturity securities, Fair Value	$ 14,194,955