Investments (Credit Quality of Fixed Maturity Security Portfolio by NAIC Designation) (Details) - USD ($) $ in Thousands	Mar. 31, 2020		Dec. 31, 2019
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	$ 47,254,063	[1]	$ 48,238,946
Fixed maturity securities, available for sale, fair value	47,704,867		51,580,490
NAIC, Class 1 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	26,309,850		27,781,525
Fixed maturity securities, available for sale, fair value	27,739,779		30,122,657
NAIC, Class 2 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	19,582,082		19,278,355
Fixed maturity securities, available for sale, fair value	18,999,760		20,316,911
NAIC, Class 3 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	1,167,772		1,001,087
Fixed maturity securities, available for sale, fair value	838,628		977,191
NAIC, Class 4 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	128,826		114,497
Fixed maturity securities, available for sale, fair value	104,090		112,534
NAIC, Class 5 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	60,550		57,952
Fixed maturity securities, available for sale, fair value	18,433		45,205
NAIC, Class 6 Designation
Investment Holdings [Line Items]
Fixed maturity securities, available for sale, amortized cost	4,983		5,530
Fixed maturity securities, available for sale, fair value	$ 4,177		$ 5,992

[1] (1) Amortized cost excludes accrued interest receivable of $456.1 million as of March 31, 2020.