Investments - Cost (amortized cost for fixed maturities and short-term investments), fair value, gross unrealized gains, gross unrealized (losses), and OTTI recorded in AOCI (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
AFS securities
Cost or amortized cost of Fixed maturities	$ 26,772,143		$ 26,544,806
Gross unrealized gains on Fixed maturities	895,557		1,584,812
Gross unrealized losses on Fixed maturities	(385,258)		(282,698)
Fixed maturities, at fair value	27,282,442		27,846,920
Gross Unrealized Losses Non-credit related OTTI	(91,977)	[1]	(119,076)	[1]
Short-term investments, amortized cost	271,465		322,563
Short-term investments, gross unrealized gains	1,300	[2]	192	[3]
Short-term investments, gross unrealized losses	(60)	[2]	(52)	[3]
Short-term investments, at fair value	272,705		322,703
Cost of equity securities	878,679		617,486
Equity securities, gross unrealized gains	113,075	[4]	31,935	[5]
Equity securities, gross unrealized losses	(17,021)	[4]	(62)	[5]
Equity securities, at fair value	974,733		649,359
Available for sale securities, Cost or amortized cost	27,922,287		27,484,855
Available for sale securities, Gross unrealized gains	1,009,932		1,616,939
Available for sale securities, Gross unrealized losses	402,339		282,812
Total investments available for sale	28,529,880		28,818,982
Fixed maturities - HTM
Fixed maturities, held to maturity	2,802,111		2,814,447
Fixed maturites held to maturity, gross unrealized gains	317,615		451,038
Fixed maturites held to maturity, gross unrealized losses	(2,786)		(2,681)
Fair value of held to maturity securities	3,116,940		3,262,804
U.S. Government and Government - Related/Supported
AFS securities
Cost or amortized cost of Fixed maturities	2,128,375	[2]	1,906,044	[3]
Gross unrealized gains on Fixed maturities	66,604	[2]	131,860	[3]
Gross unrealized losses on Fixed maturities	(18,990)	[2]	(3,287)	[3]
Fixed maturities, at fair value	2,175,989	[2]	2,034,617	[3]
Gross Unrealized Losses Non-credit related OTTI	0	[1],[2]	0	[1],[3]
Fixed maturities - HTM
Fixed maturities, held to maturity	10,712	[2]	10,788	[3]
Fixed maturites held to maturity, gross unrealized gains	946	[2]	1,651	[3]
Fixed maturites held to maturity, gross unrealized losses	0	[2]	0	[3]
Fair value of held to maturity securities	11,658	[2]	12,439	[3]
Corporate
AFS securities
Cost or amortized cost of Fixed maturities	10,395,005	[6],[7]	9,837,962	[8],[9]
Gross unrealized gains on Fixed maturities	470,327	[6],[7]	723,028	[8],[9]
Gross unrealized losses on Fixed maturities	(106,000)	[6],[7]	(78,990)	[8],[9]
Fixed maturities, at fair value	10,759,332	[6],[7]	10,482,000	[8],[9]
Gross Unrealized Losses Non-credit related OTTI	(7,945)	[1],[6],[7]	(11,453)	[1],[8],[9]
Fixed maturities - HTM
Fixed maturities, held to maturity	1,370,391		1,425,320
Fixed maturites held to maturity, gross unrealized gains	137,326		190,871
Fixed maturites held to maturity, gross unrealized losses	(1,436)		(794)
Fair value of held to maturity securities	1,506,281		1,615,397
RMBS - Agency
AFS securities
Cost or amortized cost of Fixed maturities	4,032,158		5,054,097
Gross unrealized gains on Fixed maturities	85,282		206,931
Gross unrealized losses on Fixed maturities	(48,358)		(5,535)
Fixed maturities, at fair value	4,069,082		5,255,493
Gross Unrealized Losses Non-credit related OTTI	0	[1]	0	[1]
RMBS - Non-Agency
AFS securities
Cost or amortized cost of Fixed maturities	407,789		678,469
Gross unrealized gains on Fixed maturities	30,786		46,132
Gross unrealized losses on Fixed maturities	(39,176)		(76,868)
Fixed maturities, at fair value	399,399		647,733
Gross Unrealized Losses Non-credit related OTTI	(76,138)	[1]	(93,259)	[1]
Fixed maturities - HTM
Fixed maturities, held to maturity	65,542		83,205
Fixed maturites held to maturity, gross unrealized gains	5,580		10,502
Fixed maturites held to maturity, gross unrealized losses	0		0
Fair value of held to maturity securities	71,122		93,707
CMBS
AFS securities
Cost or amortized cost of Fixed maturities	1,166,285		1,010,794
Gross unrealized gains on Fixed maturities	44,867		70,745
Gross unrealized losses on Fixed maturities	(13,271)		(4,288)
Fixed maturities, at fair value	1,197,881		1,077,251
Gross Unrealized Losses Non-credit related OTTI	(2,856)	[1]	(2,962)	[1]
Fixed maturities - HTM
Fixed maturities, held to maturity	141,884		12,751
Fixed maturites held to maturity, gross unrealized gains	14,298		2,048
Fixed maturites held to maturity, gross unrealized losses	0		0
Fair value of held to maturity securities	156,182		14,799
CDO
AFS securities
Cost or amortized cost of Fixed maturities	754,897		784,999
Gross unrealized gains on Fixed maturities	7,074		11,973
Gross unrealized losses on Fixed maturities	(49,823)		(87,156)
Fixed maturities, at fair value	712,148		709,816
Gross Unrealized Losses Non-credit related OTTI	(2,106)	[1]	(4,872)	[1]
Other asset-backed securities
AFS securities
Cost or amortized cost of Fixed maturities	1,122,958	[10]	1,426,483	[11]
Gross unrealized gains on Fixed maturities	38,343	[10]	59,663	[11]
Gross unrealized losses on Fixed maturities	(10,166)	[10]	(15,435)	[11]
Fixed maturities, at fair value	1,151,135	[10]	1,470,711	[11]
Gross Unrealized Losses Non-credit related OTTI	(2,932)	[1],[10]	(6,530)	[1],[11]
Fixed maturities - HTM
Fixed maturities, held to maturity	104,624	[10]	222,340
Fixed maturites held to maturity, gross unrealized gains	9,074	[10]	29,287	[11]
Fixed maturites held to maturity, gross unrealized losses	0	[10]	(167)	[11]
Fair value of held to maturity securities	113,698		251,460
U.S. States and political subdivisions of the States
AFS securities
Cost or amortized cost of Fixed maturities	1,784,621		1,767,669
Gross unrealized gains on Fixed maturities	61,561		146,294
Gross unrealized losses on Fixed maturities	(28,084)		(2,946)
Fixed maturities, at fair value	1,818,098		1,911,017
Gross Unrealized Losses Non-credit related OTTI	0	[1]	0	[1]
Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
AFS securities
Cost or amortized cost of Fixed maturities	4,980,055	[2]	4,078,289	[3]
Gross unrealized gains on Fixed maturities	90,713	[2]	188,186	[3]
Gross unrealized losses on Fixed maturities	(71,390)	[2]	(8,193)	[3]
Fixed maturities, at fair value	4,999,378	[2]	4,258,282	[3]
Gross Unrealized Losses Non-credit related OTTI	0	[1],[2]	0	[1],[3]
Fixed maturities - HTM
Fixed maturities, held to maturity	1,108,958	[2]	1,060,043	[3]
Fixed maturites held to maturity, gross unrealized gains	150,391	[2]	216,679	[3]
Fixed maturites held to maturity, gross unrealized losses	(1,350)	[2]	(1,720)	[3]
Fair value of held to maturity securities	$ 1,257,999	[2]	$ 1,275,002	[3]

[1] Represents the non-credit component of OTTI losses, adjusted for subsequent sales of securities. It does not include the change in fair value subsequent to the impairment measurement date.

[2] U.S. Government and Government-Related/Supported, Non-U.S. Sovereign Government, Provincials, Supranationals and Government-Related/Supported and Total short-term investments include government-related securities with an amortized cost of $2,373.4 million and fair value of $2,414.7 million and U.S. Agencies with an amortized cost of $268.7 million and fair value of $290.6 million.

[3] U.S. Government and Government-Related/Supported, Non-U.S. Sovereign Government, Provincials, Supranationals and Government-Related/Supported and Total short-term investments include government-related securities with an amortized cost of $1,912.7 million and fair value of $1,988.5 million and U.S. Agencies with an amortized cost of $404.3 million and fair value of $446.7 million.

[4] Included within Total equity securities are investments in fixed income funds with a fair value of $89.7 million and an amortized cost of $100.0 million at September 30, 2013.

[5] Included within Total equity securities are investments in fixed income funds with a fair value of $101.9 million and an amortized cost of $100.0 million at December 31, 2012.

[6] Included within Corporate are Tier One and Upper Tier Two securities, representing committed term debt and hybrid instruments, which are senior to the common and preferred equities of the financial institutions. These securities have a fair value of $271.5 million and an amortized cost of $280.2 million at September 30, 2013.

[7] Included within Corporate are certain medium term notes supported primarily by pools of European investment grade credit with varying degrees of leverage. The notes have a fair value of $150.9 million and an amortized cost of $150.6 million. These notes allow the investor to participate in cash flows of the underlying bonds including certain residual values, which could serve to either decrease or increase the ultimate values of these notes.

[8] Included within Corporate are certain medium term notes supported primarily by pools of European investment grade credit with varying degrees of leverage. The notes have a fair value of $194.3 million and an amortized cost of $194.8 million. These notes allow the investor to participate in cash flows of the underlying bonds including certain residual values, which could serve to either decrease or increase the ultimate values of these notes.

[9] Included within Corporate are Tier One and Upper Tier Two securities, representing committed term debt and hybrid instruments, which are senior to the common and preferred equities of the financial institutions. These securities have a fair value of $308.5 million and an amortized cost of $327.6 million at December 31, 2012.

[10] Covered Bonds within Fixed maturities - AFS with an amortized cost of $531.4 million and a fair value of $560.7 million and Covered Bonds within Fixed maturities - HTM with an amortized cost of $8.3 million and a fair value of $8.3 million are included within Other asset-backed securities to align the Company's classification to market indices.

[11] Covered Bonds within Fixed maturities - AFS with an amortized cost of $605.4 million and a fair value of $647.1 million and Covered Bonds within Fixed maturities - HTM with an amortized cost of $8.4 million