Investment Information - Maturity profile of available for sale securities) (Details) - USD ($) $ in Thousands		Sep. 30, 2021	Dec. 31, 2020
Estimated Fair Value
Estimated Fair Value		$ 16,768,363	$ 18,717,825
Amortized Cost
Amortized cost		16,629,621	18,143,305
Fixed maturities
Estimated Fair Value
Due in one year or less		416,601	348,200
Due after one year through five years		7,862,472	10,629,959
Due after five years through 10 years		4,483,025	4,881,564
Due after 10 years		348,490	482,180
Single maturity date		13,110,588	16,341,903
Estimated Fair Value	[1]	16,768,363	18,996,608
Amortized Cost
Due in one year or less		410,373	339,951
Due after one year through five years		7,772,773	10,340,819
Due after five years through 10 years		4,453,526	4,654,754
Due after 10 years		341,931	448,139
Single maturity date		12,978,603	15,783,663
Amortized cost	[1]	16,629,621	18,421,669
Fixed maturities | Mortgage backed securities
Estimated Fair Value
Securities without single maturity date		405,797	630,001
Estimated Fair Value		405,797	630,001
Amortized Cost
Securities without single maturity date		406,403	626,368
Amortized cost	[1]	406,403	626,368
Fixed maturities | Commercial mortgage backed securities
Estimated Fair Value
Securities without single maturity date		579,424	389,900
Estimated Fair Value		579,424	389,900
Amortized Cost
Securities without single maturity date		576,377	384,254
Amortized cost	[1]	576,377	384,254
Fixed maturities | Asset backed securities
Estimated Fair Value
Securities without single maturity date		2,672,554	1,634,804
Estimated Fair Value		2,672,554	1,634,804
Amortized Cost
Securities without single maturity date		2,668,238	1,627,384
Amortized cost	[1]	$ 2,668,238	$ 1,627,384

[1] In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities pledged. For purposes of this table, the Company has excluded the collateral received under securities lending, at fair value and included the securities pledged under securities lending, at fair value. See “—Securities Lending Agreements.” In September 2021, the Company terminated its securities lending program and no longer enters into securities lending agreements with financial institutions.