Investment Information - Summary of available for sale securities (Details) - USD ($) $ in Thousands		6 Months Ended		12 Months Ended
		Jun. 30, 2018		Dec. 31, 2017
Available for sale securities:
Estimated Fair Value		$ 15,444,258		$ 16,570,755
Gross Unrealized Gains		44,033		267,433
Gross Unrealized Losses		(207,692)		(112,447)
Cost or Amortized Cost		15,607,917		16,415,769
OTTI Unrealized Losses		(84)	[1]	(88)
Fixed maturities
Available for sale securities:
Estimated Fair Value		14,347,460		14,332,391
Gross Unrealized Gains	[2]	43,695		111,098
Gross Unrealized Losses	[2]	(207,620)		(106,181)
Cost or Amortized Cost	[2]	14,511,385		14,327,474
OTTI Unrealized Losses	[1],[2]	(84)		(88)
Fixed maturities | Corporate bonds
Available for sale securities:
Estimated Fair Value		5,535,763		4,434,439
Gross Unrealized Gains	[2]	7,199		30,943
Gross Unrealized Losses	[2]	(103,978)		(32,340)
Cost or Amortized Cost	[2]	5,632,542		4,435,836
OTTI Unrealized Losses	[1],[2]	(69)		(73)
Fixed maturities | Mortgage backed securities
Available for sale securities:
Estimated Fair Value		488,699		316,141
Gross Unrealized Gains	[2]	1,699		1,640
Gross Unrealized Losses	[2]	(4,742)		(2,561)
Cost or Amortized Cost	[2]	491,742		317,062
OTTI Unrealized Losses	[1],[2]	(15)		(15)
Fixed maturities | Municipal bonds
Available for sale securities:
Estimated Fair Value		1,431,256		2,158,840
Gross Unrealized Gains	[2]	7,371		20,285
Gross Unrealized Losses	[2]	(22,047)		(12,308)
Cost or Amortized Cost	[2]	1,445,932		2,150,863
OTTI Unrealized Losses	[1],[2]	0		0
Fixed maturities | Commercial mortgage backed securities
Available for sale securities:
Estimated Fair Value		590,198		545,817
Gross Unrealized Gains	[2]	776		2,131
Gross Unrealized Losses	[2]	(13,222)		(4,268)
Cost or Amortized Cost	[2]	602,644		547,954
OTTI Unrealized Losses	[1],[2]	0		0
Fixed maturities | US government and government agencies
Available for sale securities:
Estimated Fair Value		2,788,272		3,484,257
Gross Unrealized Gains	[2]	6,216		2,188
Gross Unrealized Losses	[2]	(19,577)		(28,769)
Cost or Amortized Cost	[2]	2,801,633		3,510,838
OTTI Unrealized Losses	[1],[2]	0		0
Fixed maturities | Non-US government securities
Available for sale securities:
Estimated Fair Value		1,692,783		1,612,754
Gross Unrealized Gains	[2]	17,124		48,764
Gross Unrealized Losses	[2]	(27,480)		(17,321)
Cost or Amortized Cost	[2]	1,703,139		1,581,311
OTTI Unrealized Losses	[1],[2]	0		0
Fixed maturities | Asset backed securities
Available for sale securities:
Estimated Fair Value		1,820,489		1,780,143
Gross Unrealized Gains	[2]	3,310		5,147
Gross Unrealized Losses	[2]	(16,574)		(8,614)
Cost or Amortized Cost	[2]	1,833,753		1,783,610
OTTI Unrealized Losses	[1],[2]	0		0
Equity securities
Available for sale securities:
Estimated Fair Value	[3]			504,333
Gross Unrealized Gains	[3]			88,739
Gross Unrealized Losses	[3]			(5,583)
Cost or Amortized Cost	[3]			421,177
OTTI Unrealized Losses	[1]			0
Other investments
Available for sale securities:
Estimated Fair Value		0		264,989
Gross Unrealized Gains		0		66,946
Gross Unrealized Losses		0		(120)
Cost or Amortized Cost		0		198,163
OTTI Unrealized Losses	[1]	0		0
Short-term investments
Available for sale securities:
Estimated Fair Value		1,096,798		1,469,042
Gross Unrealized Gains		338		650
Gross Unrealized Losses		(72)		(563)
Cost or Amortized Cost		1,096,532		1,468,955
OTTI Unrealized Losses	[1]	$ 0		$ 0

[1] Represents the total other-than-temporary impairments (“OTTI”) recognized in accumulated other comprehensive income (“AOCI”). It does not include the change in fair value subsequent to the impairment measurement date. At June 30, 2018, the net unrealized gain related to securities for which a non-credit OTTI was recognized in AOCI was $0.2 million, compared to a net unrealized gain of $0.3 million at December 31, 2017.

[2] 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.”

[3] 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.