Investment Information - Maturity profile of available for sale securities) (Details) - USD ($) $ in Thousands		Jun. 30, 2017	Dec. 31, 2016
Estimated Fair Value:
Estimated fair value		$ 13,671,011	$ 13,426,577
Amortized Cost:
Amortized cost		13,626,639	13,522,671
Fixed maturities
Estimated Fair Value:
Due in one year or less		580,006	560,830
Due after one year through five years		7,240,600	6,158,148
Due after five years through 10 years		3,542,222	4,676,847
Due after 10 years		319,402	610,962
Single maturity date		11,682,230	12,006,787
Estimated fair value	[1]	14,274,279	14,156,918
Amortized Cost:
Due in one year or less		579,694	557,675
Due after one year through five years		7,222,772	6,211,099
Due after five years through 10 years		3,528,439	4,710,017
Due after 10 years		314,853	620,849
Single maturity date		11,645,758	12,099,640
Amortized cost	[1]	14,231,622	14,256,595
Fixed maturities | Mortgage backed securities
Estimated Fair Value:
Securities without single maturity date		331,082	490,093
Amortized Cost:
Securities without single maturity date		328,028	493,656
Fixed maturities | Commercial mortgage backed securities
Estimated Fair Value:
Securities without single maturity date		521,272	536,051
Amortized Cost:
Securities without single maturity date		524,187	539,683
Fixed maturities | Asset backed securities
Estimated Fair Value:
Securities without single maturity date		1,739,695	1,123,987
Amortized Cost:
Securities without single maturity date		$ 1,733,649	$ 1,123,616

[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.”