Investment Information - Summary of available for sale securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2014		Dec. 31, 2013
Available for sale securities:
Estimated Fair Value	$ 12,335,685		$ 12,150,358
Gross Unrealized Gains	235,645		232,673
Gross Unrealized Losses	(102,260)		(152,368)
Cost or Amortized Cost	12,202,300		12,070,053
OTTI Unrealized Losses	(11,171)	[1]	(12,942)	[1]
Fixed maturities and fixed maturities pledged under securities lending agreements
Available for sale securities:
Estimated Fair Value	9,876,320	[2]	9,676,857	[2]
Gross Unrealized Gains	131,396	[2]	133,450	[2]
Gross Unrealized Losses	(78,368)	[2]	(127,100)	[2]
Cost or Amortized Cost	9,823,292	[2]	9,670,507	[2]
OTTI Unrealized Losses	(11,171)	[1],[2]	(12,942)	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds
Available for sale securities:
Estimated Fair Value	2,514,129	[2]	2,267,263	[2]
Gross Unrealized Gains	39,704	[2]	35,289	[2]
Gross Unrealized Losses	(20,047)	[2]	(35,537)	[2]
Cost or Amortized Cost	2,494,472	[2]	2,267,511	[2]
OTTI Unrealized Losses	(16)	[1],[2]	(16)	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities
Available for sale securities:
Estimated Fair Value	1,087,840	[2]	1,133,095	[2]
Gross Unrealized Gains	21,700	[2]	16,270	[2]
Gross Unrealized Losses	(14,253)	[2]	(22,209)	[2]
Cost or Amortized Cost	1,080,393	[2]	1,139,034	[2]
OTTI Unrealized Losses	(8,217)	[1],[2]	(9,269)	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds
Available for sale securities:
Estimated Fair Value	1,410,043	[2]	1,481,738	[2]
Gross Unrealized Gains	28,494	[2]	29,378	[2]
Gross Unrealized Losses	(3,742)	[2]	(9,730)	[2]
Cost or Amortized Cost	1,385,291	[2]	1,462,090	[2]
OTTI Unrealized Losses	(17)	[1],[2]	(17)	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities
Available for sale securities:
Estimated Fair Value	1,023,055	[2]	1,074,497	[2]
Gross Unrealized Gains	11,769	[2]	13,972	[2]
Gross Unrealized Losses	(9,031)	[2]	(15,224)	[2]
Cost or Amortized Cost	1,020,317	[2]	1,075,749	[2]
OTTI Unrealized Losses	0	[1],[2]	(199)	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | US government and government agencies
Available for sale securities:
Estimated Fair Value	1,153,120	[2]	1,301,809	[2]
Gross Unrealized Gains	4,048	[2]	3,779	[2]
Gross Unrealized Losses	(5,932)	[2]	(11,242)	[2]
Cost or Amortized Cost	1,155,004	[2]	1,309,272	[2]
OTTI Unrealized Losses	(19)	[1],[2]	(19)	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Non-US government securities
Available for sale securities:
Estimated Fair Value	1,114,382	[2]	1,085,861	[2]
Gross Unrealized Gains	14,538	[2]	14,729	[2]
Gross Unrealized Losses	(12,978)	[2]	(19,363)	[2]
Cost or Amortized Cost	1,112,822	[2]	1,090,495	[2]
OTTI Unrealized Losses	0	[1],[2]	0	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities
Available for sale securities:
Estimated Fair Value	1,573,751	[2]	1,332,594	[2]
Gross Unrealized Gains	11,143	[2]	20,033	[2]
Gross Unrealized Losses	(12,385)	[2]	(13,795)	[2]
Cost or Amortized Cost	1,574,993	[2]	1,326,356	[2]
OTTI Unrealized Losses	(2,902)	[1],[2]	(3,422)	[1],[2]
Equity securities
Available for sale securities:
Estimated Fair Value	548,168		496,824
Gross Unrealized Gains	78,896		69,487
Gross Unrealized Losses	(6,596)		(5,938)
Cost or Amortized Cost	475,868		433,275
OTTI Unrealized Losses	0	[1]	0	[1]
Other investments
Available for sale securities:
Estimated Fair Value	426,917		498,310
Gross Unrealized Gains	24,398		28,082
Gross Unrealized Losses	(15,243)		(18,459)
Cost or Amortized Cost	417,762		488,687
OTTI Unrealized Losses	0	[1]	0	[1]
Short-term investments
Available for sale securities:
Estimated Fair Value	1,484,280		1,478,367
Gross Unrealized Gains	955		1,654
Gross Unrealized Losses	(2,053)		(871)
Cost or Amortized Cost	1,485,378		1,477,584
OTTI Unrealized Losses	$ 0	[1]	$ 0	[1]

[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 March 31, 2014, the net unrealized gain related to securities for which a non-credit OTTI was recognized in AOCI was $4.8 million, compared to a net unrealized gain of $6.0 million at December 31, 2013.

[2] In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities and short-term investments pledged. For purposes of this table, the Company has excluded the collateral received and reinvested and included the fixed maturities and short-term investments pledged. See “—Securities Lending Agreements.”