Investments (Credit Quality of Fixed Maturity Security Portfolio by NAIC Designation) (Details) - USD ($) $ in Thousands	Jun. 30, 2020		Dec. 31, 2019
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	$ 46,951,043	[1]	$ 48,238,946
Fixed maturity securities, available for sale, fair value	50,738,792		51,580,490
NAIC, Class 1 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	25,587,355		27,781,525
Fixed maturity securities, available for sale, fair value	28,327,288		30,122,657
NAIC, Class 2 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	19,487,757		19,278,355
Fixed maturity securities, available for sale, fair value	20,765,334		20,316,911
NAIC, Class 3 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	1,577,056		1,001,087
Fixed maturity securities, available for sale, fair value	1,430,998		977,191
NAIC, Class 4 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	185,879		114,497
Fixed maturity securities, available for sale, fair value	156,234		112,534
NAIC, Class 5 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	51,893		57,952
Fixed maturity securities, available for sale, fair value	31,644		45,205
NAIC, Class 6 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	61,103		5,530
Fixed maturity securities, available for sale, fair value	$ 27,294		$ 5,992

[1] (1) Amortized cost excludes accrued interest receivable of $432.0 million as of June 30, 2020.