Investment Information - Maturity profile of available for sale securities) (Details) - USD ($) $ in Thousands		Sep. 30, 2020	Dec. 31, 2019
Estimated Fair Value
Estimated Fair Value		$ 18,452,888	$ 16,894,526
Amortized Cost
Amortized cost		17,955,515	16,598,808
Fixed maturities
Estimated Fair Value
Due in one year or less		318,767	428,659
Due after one year through five years		9,937,129	10,126,403
Due after five years through 10 years		4,849,248	3,317,535
Due after 10 years		551,860	411,269
Single maturity date		15,657,004	14,283,866
Estimated Fair Value	[1]	18,515,637	17,263,036
Amortized Cost
Due in one year or less		312,569	423,617
Due after one year through five years		9,698,743	9,996,206
Due after five years through 10 years		4,641,483	3,219,567
Due after 10 years		525,863	388,280
Single maturity date		15,178,658	14,027,670
Amortized cost	[1]	18,018,177	16,966,188
Fixed maturities | Mortgage backed securities
Estimated Fair Value
Securities without single maturity date		717,081	562,309
Estimated Fair Value		717,081	562,309
Amortized Cost
Securities without single maturity date		708,334	553,571
Amortized cost	[1]	708,334	553,571
Fixed maturities | Commercial mortgage backed securities
Estimated Fair Value
Securities without single maturity date		374,360	733,108
Estimated Fair Value		374,360	733,108
Amortized Cost
Securities without single maturity date		370,753	720,487
Amortized cost	[1]	370,753	720,487
Fixed maturities | Asset backed securities
Estimated Fair Value
Securities without single maturity date		1,767,192	1,683,753
Estimated Fair Value		1,767,192	1,683,753
Amortized Cost
Securities without single maturity date		1,760,432	1,664,460
Amortized cost	[1]	$ 1,760,432	$ 1,664,460

[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.”