Investment Information - Aging of available for sale securities in an unrealized loss position (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Available for sale securities:
Estimated Fair Value - Less than 12 Months		$ 10,635,259	$ 9,064,672
Gross Unrealized Losses - Less than 12 Months		(181,808)	(91,691)
Estimated Fair Value - 12 Months or More		715,345	911,838
Gross Unrealized Losses - 12 Months or More		(25,884)	(20,756)
Estimated Fair Value - Total		11,350,604	9,976,510
Gross Unrealized Losses - Total		(207,692)	(112,447)
Fixed maturities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	10,581,322	8,773,557
Gross Unrealized Losses - Less than 12 Months	[1]	(181,736)	(85,425)
Estimated Fair Value - 12 Months or More	[1]	715,345	911,838
Gross Unrealized Losses - 12 Months or More	[1]	(25,884)	(20,756)
Estimated Fair Value - Total	[1]	11,296,667	9,685,395
Gross Unrealized Losses - Total	[1]	(207,620)	(106,181)
Fixed maturities | Corporate bonds
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	4,541,077	2,320,716
Gross Unrealized Losses - Less than 12 Months	[1]	(94,563)	(25,411)
Estimated Fair Value - 12 Months or More	[1]	236,332	279,082
Gross Unrealized Losses - 12 Months or More	[1]	(9,415)	(6,929)
Estimated Fair Value - Total	[1]	4,777,409	2,599,798
Gross Unrealized Losses - Total	[1]	(103,978)	(32,340)
Fixed maturities | Mortgage backed securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	296,876	221,113
Gross Unrealized Losses - Less than 12 Months	[1]	(4,618)	(1,715)
Estimated Fair Value - 12 Months or More	[1]	708	28,380
Gross Unrealized Losses - 12 Months or More	[1]	(124)	(846)
Estimated Fair Value - Total	[1]	297,584	249,493
Gross Unrealized Losses - Total	[1]	(4,742)	(2,561)
Fixed maturities | Municipal bonds
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	814,694	1,030,389
Gross Unrealized Losses - Less than 12 Months	[1]	(16,888)	(8,438)
Estimated Fair Value - 12 Months or More	[1]	109,913	132,469
Gross Unrealized Losses - 12 Months or More	[1]	(5,159)	(3,870)
Estimated Fair Value - Total	[1]	924,607	1,162,858
Gross Unrealized Losses - Total	[1]	(22,047)	(12,308)
Fixed maturities | Commercial mortgage backed securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	351,013	225,164
Gross Unrealized Losses - Less than 12 Months	[1]	(9,067)	(1,899)
Estimated Fair Value - 12 Months or More	[1]	56,382	57,291
Gross Unrealized Losses - 12 Months or More	[1]	(4,155)	(2,369)
Estimated Fair Value - Total	[1]	407,395	282,455
Gross Unrealized Losses - Total	[1]	(13,222)	(4,268)
Fixed maturities | US government and government agencies
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	1,957,338	2,646,415
Gross Unrealized Losses - Less than 12 Months	[1]	(18,470)	(26,501)
Estimated Fair Value - 12 Months or More	[1]	41,069	111,879
Gross Unrealized Losses - 12 Months or More	[1]	(1,107)	(2,268)
Estimated Fair Value - Total	[1]	1,998,407	2,758,294
Gross Unrealized Losses - Total	[1]	(19,577)	(28,769)
Fixed maturities | Non-US government securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	1,243,055	1,218,514
Gross Unrealized Losses - Less than 12 Months	[1]	(24,034)	(15,546)
Estimated Fair Value - 12 Months or More	[1]	160,434	93,530
Gross Unrealized Losses - 12 Months or More	[1]	(3,446)	(1,775)
Estimated Fair Value - Total	[1]	1,403,489	1,312,044
Gross Unrealized Losses - Total	[1]	(27,480)	(17,321)
Fixed maturities | Asset backed securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	1,377,269	1,111,246
Gross Unrealized Losses - Less than 12 Months	[1]	(14,096)	(5,915)
Estimated Fair Value - 12 Months or More	[1]	110,507	209,207
Gross Unrealized Losses - 12 Months or More	[1]	(2,478)	(2,699)
Estimated Fair Value - Total	[1]	1,487,776	1,320,453
Gross Unrealized Losses - Total	[1]	(16,574)	(8,614)
Equity securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[2]		166,562
Gross Unrealized Losses - Less than 12 Months	[2]		(5,583)
Estimated Fair Value - 12 Months or More	[2]		0
Gross Unrealized Losses - 12 Months or More	[2]		0
Estimated Fair Value - Total	[2]		166,562
Gross Unrealized Losses - Total	[2]		(5,583)
Other investments
Available for sale securities:
Estimated Fair Value - Less than 12 Months		0	15,025
Gross Unrealized Losses - Less than 12 Months		0	(120)
Estimated Fair Value - 12 Months or More		0	0
Gross Unrealized Losses - 12 Months or More		0	0
Estimated Fair Value - Total		0	15,025
Gross Unrealized Losses - Total		0	(120)
Short-term investments
Available for sale securities:
Estimated Fair Value - Less than 12 Months		53,937	109,528
Gross Unrealized Losses - Less than 12 Months		(72)	(563)
Estimated Fair Value - 12 Months or More		0	0
Gross Unrealized Losses - 12 Months or More		0	0
Estimated Fair Value - Total		53,937	109,528
Gross Unrealized Losses - Total		$ (72)	$ (563)

[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.”

[2] Effective January 1, 2018, the Company adopted new accounting guidance for financial instruments (see Note 2). As a result, equity securities are no longer accounted for as available for sale and are excluded from this table.