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	Jun. 30, 2013		Dec. 31, 2012
AFS securities
Cost or amortized cost of Fixed maturities	$ 26,016,208		$ 26,544,806
Gross unrealized gains, fixed maturities	877,540		1,584,812
Gross unrealized losses related to changes in estimated fair value, fixed maturities	(438,645)		(282,698)
Fixed maturities, at fair value	26,455,103		27,846,920
Gross Unrealized Losses Non-credit related OTTI	(99,451)	[1]	(119,076)	[1]
Short-term investments, amortized cost	234,775		322,563
Short-term investments, gross unrealized gains	214	[2]	192	[2]
Short-term investments, gross unrealized losses related to changes in estimated fair value	(537)	[2]	(52)	[2]
Short-term investments, at fair value	234,452		322,703
Cost of equity securities	873,465		617,486
Equity securities, gross unrealized gains	64,450	[3]	31,935	[3]
Equity securities, gross unrealized losses related to changes in estimated fair value	(21,907)	[3]	(62)	[3]
Equity securities, at fair value	916,008		649,359
Available-for-sale Securities, Amortized Cost Basis	27,124,448		27,484,855
Gross unrealized gains, available for sale	942,204		1,616,939
Gross unrealized losses related to changes in estimated fair value, available for sale	(461,089)		(282,812)
Total investments available for sale	27,605,563		28,818,982
Fixed maturities - HTM
Fixed maturities, held to maturity	2,686,994		2,814,447
Gross unrealized gains, held to maturity	295,248		451,038
Gross unrealized losses related to changes in estimated fair value, held to maturity	(3,261)		(2,681)
Fair value of held to maturity securities	2,978,981		3,262,804
U.S. Government and Government - Related/Supported
AFS securities
Cost or amortized cost of Fixed maturities	1,971,139	[2]	1,906,044	[2]
Gross unrealized gains, fixed maturities	71,410	[2]	131,860	[2]
Gross unrealized losses related to changes in estimated fair value, fixed maturities	(19,727)	[2]	(3,287)	[2]
Fixed maturities, at fair value	2,022,822	[2]	2,034,617	[2]
Gross Unrealized Losses Non-credit related OTTI	0	[1],[2]	0	[1],[2]
Fixed maturities - HTM
Fixed maturities, held to maturity	10,241		10,788
Gross unrealized gains, held to maturity	985		1,651
Gross unrealized losses related to changes in estimated fair value, held to maturity	0		0
Fair value of held to maturity securities	11,226		12,439
Corporate
AFS securities
Cost or amortized cost of Fixed maturities	9,752,465	[4],[5]	9,837,962	[4],[5]
Gross unrealized gains, fixed maturities	451,524	[4],[5]	723,028	[4],[5]
Gross unrealized losses related to changes in estimated fair value, fixed maturities	(124,514)	[4],[5]	(78,990)	[4],[5]
Fixed maturities, at fair value	10,079,475	[4],[5]	10,482,000	[4],[5]
Gross Unrealized Losses Non-credit related OTTI	(11,417)	[1],[4],[5]	(11,453)	[1],[4],[5]
Fixed maturities - HTM
Fixed maturities, held to maturity	1,312,332		1,425,320
Gross unrealized gains, held to maturity	118,136		190,871
Gross unrealized losses related to changes in estimated fair value, held to maturity	(2,122)		(794)
Fair value of held to maturity securities	1,428,346		1,615,397
RMBS - Agency
AFS securities
Cost or amortized cost of Fixed maturities	4,411,540		5,054,097
Gross unrealized gains, fixed maturities	90,119		206,931
Gross unrealized losses related to changes in estimated fair value, fixed maturities	(54,934)		(5,535)
Fixed maturities, at fair value	4,446,725		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	418,458		678,469
Gross unrealized gains, fixed maturities	28,161		46,132
Gross unrealized losses related to changes in estimated fair value, fixed maturities	(40,217)		(76,868)
Fixed maturities, at fair value	406,402		647,733
Gross Unrealized Losses Non-credit related OTTI	(77,252)	[1]	(93,259)	[1]
Fixed maturities - HTM
Fixed maturities, held to maturity	62,925		83,205
Gross unrealized gains, held to maturity	4,824		10,502
Gross unrealized losses related to changes in estimated fair value, held to maturity	0		0
Fair value of held to maturity securities	67,749		93,707
CMBS
AFS securities
Cost or amortized cost of Fixed maturities	1,093,641		1,010,794
Gross unrealized gains, fixed maturities	49,889		70,745
Gross unrealized losses related to changes in estimated fair value, fixed maturities	(14,458)		(4,288)
Fixed maturities, at fair value	1,129,072		1,077,251
Gross Unrealized Losses Non-credit related OTTI	(2,911)	[1]	(2,962)	[1]
Fixed maturities - HTM
Fixed maturities, held to maturity	136,193		12,751
Gross unrealized gains, held to maturity	14,739		2,048
Gross unrealized losses related to changes in estimated fair value, held to maturity	0		0
Fair value of held to maturity securities	150,932		14,799
CDO
AFS securities
Cost or amortized cost of Fixed maturities	753,601		784,999
Gross unrealized gains, fixed maturities	8,992		11,973
Gross unrealized losses related to changes in estimated fair value, fixed maturities	(48,345)		(87,156)
Fixed maturities, at fair value	714,248		709,816
Gross Unrealized Losses Non-credit related OTTI	(4,840)	[1]	(4,872)	[1]
Other asset-backed securities
AFS securities
Cost or amortized cost of Fixed maturities	1,165,383	[6]	1,426,483	[6]
Gross unrealized gains, fixed maturities	36,557	[6]	59,663	[6]
Gross unrealized losses related to changes in estimated fair value, fixed maturities	(13,180)	[6]	(15,435)	[6]
Fixed maturities, at fair value	1,188,760	[6]	1,470,711	[6]
Gross Unrealized Losses Non-credit related OTTI	(3,031)	[1],[6]	(6,530)	[1],[6]
Fixed maturities - HTM
Fixed maturities, held to maturity	100,551	[6]	222,340	[6]
Gross unrealized gains, held to maturity	8,225	[6]	29,287	[6]
Gross unrealized losses related to changes in estimated fair value, held to maturity	(214)	[6]	(167)	[6]
Fair value of held to maturity securities	108,562	[6]	251,460	[6]
U.S. States and political subdivisions of the States
AFS securities
Cost or amortized cost of Fixed maturities	1,758,251		1,767,669
Gross unrealized gains, fixed maturities	64,741		146,294
Gross unrealized losses related to changes in estimated fair value, fixed maturities	(24,725)		(2,946)
Fixed maturities, at fair value	1,798,267		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	4,691,730	[2]	4,078,289	[2]
Gross unrealized gains, fixed maturities	76,147	[2]	188,186	[2]
Gross unrealized losses related to changes in estimated fair value, fixed maturities	(98,545)	[2]	(8,193)	[2]
Fixed maturities, at fair value	4,669,332	[2]	4,258,282	[2]
Gross Unrealized Losses Non-credit related OTTI	0	[1],[2]	0	[1],[2]
Fixed maturities - HTM
Fixed maturities, held to maturity	1,064,752	[2]	1,060,043	[2]
Gross unrealized gains, held to maturity	148,339	[2]	216,679	[2]
Gross unrealized losses related to changes in estimated fair value, held to maturity	(925)	[2]	(1,720)	[2]
Fair value of held to maturity securities	$ 1,212,166	[2]	$ 1,275,002	[2]

[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 includes government-related securities with an amortized cost of $2,357.0 million and fair value of $2,374.5 million and U.S. Agencies with an amortized cost of $297.2 million and fair value of $321.6 million.

[3] Included within Total equity securities are investments in fixed income funds with a fair value of $91.3 million and an amortized cost of $100.0 million at June 30, 2013.

[4] 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 $149.4 million and an amortized cost of $150.2 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.

[5] 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 $281.4 million and an amortized cost of $298.6 million at June 30, 2013.

[6] Covered Bonds within Fixed maturities - AFS with an amortized cost of $523.5 million and a fair value of $550.9 million and Covered Bonds within Fixed maturities - HTM with an amortized cost of $8.0 million and a fair value of $8.2 million are included within Other asset-backed securities to align the Company's classification to market indices.