Investment Information - Aging of available for sale securities in an unrealized loss position (Details) - USD ($) $ in Thousands		Jun. 30, 2017	Dec. 31, 2016
Available for sale securities:
Estimated Fair Value - Less than 12 Months		$ 7,236,411	$ 7,509,193
Gross Unrealized Losses - Less than 12 Months		(87,843)	(192,881)
Estimated Fair Value - 12 Months or More		83,471	154,001
Gross Unrealized Losses - 12 Months or More		(2,145)	(6,005)
Estimated Fair Value - Total		7,319,882	7,663,194
Gross Unrealized Losses - Total		(89,988)	(198,886)
Fixed maturities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	7,044,732	7,171,367
Gross Unrealized Losses - Less than 12 Months	[1]	(80,493)	(172,754)
Estimated Fair Value - 12 Months or More	[1]	83,471	154,001
Gross Unrealized Losses - 12 Months or More	[1]	(2,145)	(6,005)
Estimated Fair Value - Total	[1]	7,128,203	7,325,368
Gross Unrealized Losses - Total	[1]	(82,638)	(178,759)
Fixed maturities | Corporate bonds
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	1,384,481	1,700,813
Gross Unrealized Losses - Less than 12 Months	[1]	(22,572)	(43,011)
Estimated Fair Value - 12 Months or More	[1]	15,198	46,902
Gross Unrealized Losses - 12 Months or More	[1]	(1,060)	(3,894)
Estimated Fair Value - Total	[1]	1,399,679	1,747,715
Gross Unrealized Losses - Total	[1]	(23,632)	(46,905)
Fixed maturities | Mortgage backed securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	189,816	402,699
Gross Unrealized Losses - Less than 12 Months	[1]	(1,534)	(8,134)
Estimated Fair Value - 12 Months or More	[1]	1,573	6,105
Gross Unrealized Losses - 12 Months or More	[1]	(62)	(223)
Estimated Fair Value - Total	[1]	191,389	408,804
Gross Unrealized Losses - Total	[1]	(1,596)	(8,357)
Fixed maturities | Municipal bonds
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	788,714	1,513,308
Gross Unrealized Losses - Less than 12 Months	[1]	(9,329)	(28,504)
Estimated Fair Value - 12 Months or More	[1]	39,912	29,636
Gross Unrealized Losses - 12 Months or More	[1]	(512)	(650)
Estimated Fair Value - Total	[1]	828,626	1,542,944
Gross Unrealized Losses - Total	[1]	(9,841)	(29,154)
Fixed maturities | Commercial mortgage backed securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	249,658	231,374
Gross Unrealized Losses - Less than 12 Months	[1]	(6,232)	(6,331)
Estimated Fair Value - 12 Months or More	[1]	3,383	5,635
Gross Unrealized Losses - 12 Months or More	[1]	(131)	(177)
Estimated Fair Value - Total	[1]	253,041	237,009
Gross Unrealized Losses - Total	[1]	(6,363)	(6,508)
Fixed maturities | US government and government agencies
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	2,662,818	1,888,018
Gross Unrealized Losses - Less than 12 Months	[1]	(15,246)	(24,437)
Estimated Fair Value - 12 Months or More	[1]	0	0
Gross Unrealized Losses - 12 Months or More	[1]	0	0
Estimated Fair Value - Total	[1]	2,662,818	1,888,018
Gross Unrealized Losses - Total	[1]	(15,246)	(24,437)
Fixed maturities | Non-US government securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	1,017,781	807,598
Gross Unrealized Losses - Less than 12 Months	[1]	(21,360)	(56,872)
Estimated Fair Value - 12 Months or More	[1]	11,123	0
Gross Unrealized Losses - 12 Months or More	[1]	(106)	0
Estimated Fair Value - Total	[1]	1,028,904	807,598
Gross Unrealized Losses - Total	[1]	(21,466)	(56,872)
Fixed maturities | Asset backed securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	[1]	751,464	627,557
Gross Unrealized Losses - Less than 12 Months	[1]	(4,220)	(5,465)
Estimated Fair Value - 12 Months or More	[1]	12,282	65,723
Gross Unrealized Losses - 12 Months or More	[1]	(274)	(1,061)
Estimated Fair Value - Total	[1]	763,746	693,280
Gross Unrealized Losses - Total	[1]	(4,494)	(6,526)
Equity securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months		167,176	269,381
Gross Unrealized Losses - Less than 12 Months		(7,200)	(17,517)
Estimated Fair Value - 12 Months or More		0	0
Gross Unrealized Losses - 12 Months or More		0	0
Estimated Fair Value - Total		167,176	269,381
Gross Unrealized Losses - Total		(7,200)	(17,517)
Other investments
Available for sale securities:
Estimated Fair Value - Less than 12 Months		1,562	39,299
Gross Unrealized Losses - Less than 12 Months		(28)	(2,465)
Estimated Fair Value - 12 Months or More		0	0
Gross Unrealized Losses - 12 Months or More		0	0
Estimated Fair Value - Total		1,562	39,299
Gross Unrealized Losses - Total		(28)	(2,465)
Short-term investments
Available for sale securities:
Estimated Fair Value - Less than 12 Months		22,941	29,146
Gross Unrealized Losses - Less than 12 Months		(122)	(145)
Estimated Fair Value - 12 Months or More		0	0
Gross Unrealized Losses - 12 Months or More		0	0
Estimated Fair Value - Total		22,941	29,146
Gross Unrealized Losses - Total		$ (122)	$ (145)

[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.”