Investment Information - Summary of available for sale securities (Details) - USD ($) $ in Thousands		6 Months Ended	12 Months Ended
		Jun. 30, 2017	Dec. 31, 2016
Available for sale securities:
Estimated Fair Value		$ 15,905,166	$ 15,469,573
Gross Unrealized Gains		245,442	163,339
Gross Unrealized Losses		(89,988)	(198,886)
Cost or Amortized Cost		15,749,712	15,505,120
OTTI Unrealized Losses	[1]	(3,592)	(5,831)
Fixed maturities
Available for sale securities:
Estimated Fair Value		14,274,279	14,156,918
Gross Unrealized Gains	[2]	125,295	79,082
Gross Unrealized Losses	[2]	(82,638)	(178,759)
Cost or Amortized Cost	[2]	14,231,622	14,256,595
OTTI Unrealized Losses	[1],[2]	(3,592)	(5,831)
Fixed maturities | Corporate bonds
Available for sale securities:
Estimated Fair Value		4,262,411	4,392,373
Gross Unrealized Gains	[2]	42,142	27,606
Gross Unrealized Losses	[2]	(23,632)	(46,905)
Cost or Amortized Cost	[2]	4,243,901	4,411,672
OTTI Unrealized Losses	[1],[2]	(468)	(2,285)
Fixed maturities | Mortgage backed securities
Available for sale securities:
Estimated Fair Value		331,082	490,093
Gross Unrealized Gains	[2]	4,650	4,794
Gross Unrealized Losses	[2]	(1,596)	(8,357)
Cost or Amortized Cost	[2]	328,028	493,656
OTTI Unrealized Losses	[1],[2]	(3,102)	(3,323)
Fixed maturities | Municipal bonds
Available for sale securities:
Estimated Fair Value		2,618,827	3,713,434
Gross Unrealized Gains	[2]	29,728	8,554
Gross Unrealized Losses	[2]	(9,841)	(29,154)
Cost or Amortized Cost	[2]	2,598,940	3,734,034
OTTI Unrealized Losses	[1],[2]	0	(201)
Fixed maturities | Commercial mortgage backed securities
Available for sale securities:
Estimated Fair Value		521,272	536,051
Gross Unrealized Gains	[2]	3,448	2,876
Gross Unrealized Losses	[2]	(6,363)	(6,508)
Cost or Amortized Cost	[2]	524,187	539,683
OTTI Unrealized Losses	[1],[2]	0	0
Fixed maturities | US government and government agencies
Available for sale securities:
Estimated Fair Value		3,425,196	2,804,540
Gross Unrealized Gains	[2]	4,673	9,319
Gross Unrealized Losses	[2]	(15,246)	(24,437)
Cost or Amortized Cost	[2]	3,435,769	2,819,658
OTTI Unrealized Losses	[1],[2]	0	0
Fixed maturities | Non-US government securities
Available for sale securities:
Estimated Fair Value		1,375,796	1,096,440
Gross Unrealized Gains	[2]	30,114	19,036
Gross Unrealized Losses	[2]	(21,466)	(56,872)
Cost or Amortized Cost	[2]	1,367,148	1,134,276
OTTI Unrealized Losses	[1],[2]	0	0
Fixed maturities | Asset backed securities
Available for sale securities:
Estimated Fair Value		1,739,695	1,123,987
Gross Unrealized Gains	[2]	10,540	6,897
Gross Unrealized Losses	[2]	(4,494)	(6,526)
Cost or Amortized Cost	[2]	1,733,649	1,123,616
OTTI Unrealized Losses	[1],[2]	(22)	(22)
Equity securities
Available for sale securities:
Estimated Fair Value		467,870	532,680
Gross Unrealized Gains		79,953	62,627
Gross Unrealized Losses		(7,200)	(17,517)
Cost or Amortized Cost		395,117	487,570
OTTI Unrealized Losses	[1]	0	0
Other investments
Available for sale securities:
Estimated Fair Value		248,661	167,970
Gross Unrealized Gains		39,748	21,358
Gross Unrealized Losses		(28)	(2,465)
Cost or Amortized Cost		208,941	149,077
OTTI Unrealized Losses	[1]	0	0
Short-term investments
Available for sale securities:
Estimated Fair Value		914,356	612,005
Gross Unrealized Gains		446	272
Gross Unrealized Losses		(122)	(145)
Cost or Amortized Cost		914,032	611,878
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, 2017, the net unrealized gain related to securities for which a non-credit OTTI was recognized in AOCI was $0.8 million, compared to a net unrealized gain of $2.8 million at December 31, 2016.

[2] 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.”