Investments (Credit Quality of Fixed Maturity Security Portfolio by NAIC Designation) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	$ 47,477,956	$ 46,131,190
Fixed maturity securities, available for sale, fair value	50,171,303	45,923,727
NAIC, Class 1 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	27,355,251	26,588,352
Fixed maturity securities, available for sale, fair value	29,363,924	26,921,843
NAIC, Class 2 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	18,674,100	17,901,161
Fixed maturity securities, available for sale, fair value	19,421,029	17,528,072
NAIC, Class 3 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	1,250,071	1,396,650
Fixed maturity securities, available for sale, fair value	1,213,115	1,269,242
NAIC, Class 4 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	193,004	173,987
Fixed maturity securities, available for sale, fair value	166,568	137,991
NAIC, Class 5 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	0	23,836
Fixed maturity securities, available for sale, fair value	0	19,453
NAIC, Class 6 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	5,530	47,204
Fixed maturity securities, available for sale, fair value	$ 6,667	$ 47,126