Investments (Credit Quality of Fixed Maturity Security Portfolio by NAIC Designation) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	$ 46,727,050	$ 46,131,190
Fixed maturity securities, available for sale, fair value	48,037,107	45,923,727
NAIC, Class 1 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	26,735,031	26,588,352
Fixed maturity securities, available for sale, fair value	27,929,128	26,921,843
NAIC, Class 2 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	18,402,270	17,901,161
Fixed maturity securities, available for sale, fair value	18,615,641	17,528,072
NAIC, Class 3 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	1,346,654	1,396,650
Fixed maturity securities, available for sale, fair value	1,275,240	1,269,242
NAIC, Class 4 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	172,012	173,987
Fixed maturity securities, available for sale, fair value	147,717	137,991
NAIC, Class 5 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	5,061	23,836
Fixed maturity securities, available for sale, fair value	5,017	19,453
NAIC, Class 6 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	66,022	47,204
Fixed maturity securities, available for sale, fair value	$ 64,364	$ 47,126