Investments (Credit Quality of Fixed Maturity Security Portfolio by NAIC Designation) (Details) - USD ($) $ in Thousands	Sep. 30, 2020		Dec. 31, 2019
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	$ 47,132,615	[1]	$ 48,238,946
Fixed maturity securities, available for sale, fair value	51,700,743		51,580,490
NAIC, Class 1 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	25,857,683		27,781,525
Fixed maturity securities, available for sale, fair value	28,981,733		30,122,657
NAIC, Class 2 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	18,973,077		19,278,355
Fixed maturity securities, available for sale, fair value	20,682,645		20,316,911
NAIC, Class 3 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	1,844,263		1,001,087
Fixed maturity securities, available for sale, fair value	1,684,689		977,191
NAIC, Class 4 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	300,794		114,497
Fixed maturity securities, available for sale, fair value	248,133		112,534
NAIC, Class 5 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	81,869		57,952
Fixed maturity securities, available for sale, fair value	80,048		45,205
NAIC, Class 6 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	74,929		5,530
Fixed maturity securities, available for sale, fair value	$ 23,495		$ 5,992

[1] (1) Amortized cost excludes accrued interest receivable of $434.3 million as of September 30, 2020.