Investment Information - Maturity profile of available for sale securities) (Details) - USD ($) $ in Thousands		Sep. 30, 2019	Dec. 31, 2018
Estimated Fair Value:
Estimated fair value		$ 16,470,523	$ 14,699,010
Amortized Cost:
Amortized cost		16,146,333	14,829,902
Fixed maturities
Estimated Fair Value:
Due in one year or less		545,056	276,682
Due after one year through five years		10,096,867	8,666,297
Due after five years through 10 years		3,075,318	2,919,232
Due after 10 years		207,244	218,768
Single maturity date		13,924,485	12,080,979
Estimated fair value	[1]	16,880,318	14,952,510
Amortized Cost:
Due in one year or less		538,903	279,135
Due after one year through five years		9,971,896	8,738,944
Due after five years through 10 years		2,952,478	2,951,582
Due after 10 years		197,439	226,537
Single maturity date		13,660,716	12,196,198
Amortized cost	[1]	16,555,010	15,081,793
Fixed maturities | Mortgage backed securities
Estimated Fair Value:
Securities without single maturity date		544,033	541,193
Amortized Cost:
Securities without single maturity date		533,766	540,418
Fixed maturities | Commercial mortgage backed securities
Estimated Fair Value:
Securities without single maturity date		754,306	729,442
Amortized Cost:
Securities without single maturity date		726,941	737,543
Fixed maturities | Asset backed securities
Estimated Fair Value:
Securities without single maturity date		1,657,494	1,600,896
Amortized Cost:
Securities without single maturity date		$ 1,633,587	$ 1,607,634

[1] In securities lending transactions, the Company receives collateral in excess of the fair value of the securities 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.”