Fair Value Measurements (Detail) - Schedule of assets and liabilities that were accounted for at fair value (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012		Dec. 31, 2011
Assets
Fixed maturities, at fair value	$ 27,291,662	[1]	$ 26,190,025	[1],[2]
Equity securities, at fair value	631,068	[3]	468,197	[3]
Short-term investments, at fair value	212,327	[4],[5]	359,063	[4],[5]
Total investments available for sale	28,135,057		27,017,285
Cash equivalents	1,707,035	[6]	2,754,365	[6]
Other investments	843,607	[7]	661,557	[7]
Other assets	38,338	[8],[9]	65,734	[8],[9]
Total assets accounted for at fair value	30,724,037		30,498,941
Liabilities
Financial instruments sold, but not yet purchased	21,575	[10]	20,844	[10]
Other liabilities	66,366	[8],[9]	58,706	[8],[9]
Total liabilities accounted for at fair value	87,941		79,550
US Government-sponsored Enterprises Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Assets
Fixed maturities, at fair value	0		0
US Government-sponsored Enterprises Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Assets
Fixed maturities, at fair value	2,233,820		1,990,983
US Government-sponsored Enterprises Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets
Fixed maturities, at fair value	0		0
US Government-sponsored Enterprises Debt Securities [Member] | Collateral and Counterparty Netting [Member]
Assets
Fixed maturities, at fair value	0		0
US Government-sponsored Enterprises Debt Securities [Member]
Assets
Fixed maturities, at fair value	2,233,820		1,990,983
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Assets
Fixed maturities, at fair value	0	[2],[4]	0	[2],[4]
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Assets
Fixed maturities, at fair value	10,186,687	[2],[4]	10,084,804	[2],[4]
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets
Fixed maturities, at fair value	33,305	[2],[4]	23,818	[2],[4]
Corporate Debt Securities [Member] | Collateral and Counterparty Netting [Member]
Assets
Fixed maturities, at fair value	0	[2],[4]	0	[2],[4]
Corporate Debt Securities [Member]
Assets
Fixed maturities, at fair value	10,219,992	[11],[12],[2],[4]	10,108,622	[13],[14],[15],[2],[4]
Residential Mortgage Backed Securities [Member] | Agency Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Assets
Fixed maturities, at fair value	0		0
Residential Mortgage Backed Securities [Member] | Agency Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Assets
Fixed maturities, at fair value	5,353,596		5,347,365
Residential Mortgage Backed Securities [Member] | Agency Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets
Fixed maturities, at fair value	39,795		32,041
Residential Mortgage Backed Securities [Member] | Agency Securities [Member] | Collateral and Counterparty Netting [Member]
Assets
Fixed maturities, at fair value	0		0
Residential Mortgage Backed Securities [Member] | Agency Securities [Member]
Assets
Fixed maturities, at fair value	5,393,391	[1]	5,379,406	[1]
Residential Mortgage Backed Securities [Member] | Non-Agency [Member] | Fair Value, Inputs, Level 1 [Member]
Assets
Fixed maturities, at fair value	0		0
Residential Mortgage Backed Securities [Member] | Non-Agency [Member] | Fair Value, Inputs, Level 2 [Member]
Assets
Fixed maturities, at fair value	632,054		641,815
Residential Mortgage Backed Securities [Member] | Non-Agency [Member] | Fair Value, Inputs, Level 3 [Member]
Assets
Fixed maturities, at fair value	0		0
Residential Mortgage Backed Securities [Member] | Non-Agency [Member] | Collateral and Counterparty Netting [Member]
Assets
Fixed maturities, at fair value	0		0
Residential Mortgage Backed Securities [Member] | Non-Agency [Member]
Assets
Fixed maturities, at fair value	632,054	[1]	641,815	[1]
Other Asset Backed Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Assets
Fixed maturities, at fair value	0	[2]	0	[2]
Other Asset Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Assets
Fixed maturities, at fair value	1,409,732	[2]	1,323,697	[2]
Other Asset Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets
Fixed maturities, at fair value	16,410	[2]	16,552	[2]
Other Asset Backed Securities [Member] | Collateral and Counterparty Netting [Member]
Assets
Fixed maturities, at fair value	0	[2]	0	[2]
Other Asset Backed Securities [Member]
Assets
Fixed maturities, at fair value	1,426,142	[2]	1,340,249	[13],[2]
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Assets
Fixed maturities, at fair value	0		0
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Assets
Fixed maturities, at fair value	1,768,182		1,797,378
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets
Fixed maturities, at fair value	0		0
US States and Political Subdivisions Debt Securities [Member] | Collateral and Counterparty Netting [Member]
Assets
Fixed maturities, at fair value	0		0
US States and Political Subdivisions Debt Securities [Member]
Assets
Fixed maturities, at fair value	1,768,182		1,797,378
Foreign Government Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Assets
Fixed maturities, at fair value	0		0
Foreign Government Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Assets
Fixed maturities, at fair value	4,027,363		3,298,135
Foreign Government Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets
Fixed maturities, at fair value	0		0
Foreign Government Debt Securities [Member] | Collateral and Counterparty Netting [Member]
Assets
Fixed maturities, at fair value	0		0
Foreign Government Debt Securities [Member]
Assets
Fixed maturities, at fair value	4,027,363		3,298,135
Commercial Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Assets
Fixed maturities, at fair value	0		0
Commercial Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Assets
Fixed maturities, at fair value	944,153		974,835
Commercial Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets
Fixed maturities, at fair value	0		0
Commercial Mortgage Backed Securities [Member] | Collateral and Counterparty Netting [Member]
Assets
Fixed maturities, at fair value	0		0
Commercial Mortgage Backed Securities [Member]
Assets
Fixed maturities, at fair value	944,153	[1]	974,835	[1]
Collateralized Debt Obligations [Member] | Fair Value, Inputs, Level 1 [Member]
Assets
Fixed maturities, at fair value	0		0
Collateralized Debt Obligations [Member] | Fair Value, Inputs, Level 2 [Member]
Assets
Fixed maturities, at fair value	7,868		7,751
Collateralized Debt Obligations [Member] | Fair Value, Inputs, Level 3 [Member]
Assets
Fixed maturities, at fair value	638,697		650,851
Collateralized Debt Obligations [Member] | Collateral and Counterparty Netting [Member]
Assets
Fixed maturities, at fair value	0		0
Collateralized Debt Obligations [Member]
Assets
Fixed maturities, at fair value	646,565	[1]	658,602	[1]
Fair Value, Inputs, Level 1 [Member]
Assets
Fixed maturities, at fair value	0		0	[2]
Equity securities, at fair value	322,713	[3]	239,175	[3]
Short-term investments, at fair value	0	[4],[5]	0	[4],[5]
Total investments available for sale	322,713		239,175
Cash equivalents	1,140,591	[6]	1,686,101	[6]
Other investments	0	[7]	0	[7]
Other assets	0	[8],[9]	0	[8],[9]
Total assets accounted for at fair value	1,463,304		1,925,276
Liabilities
Financial instruments sold, but not yet purchased	0	[10]	0	[10]
Other liabilities	0	[8],[9]	0	[8],[9]
Total liabilities accounted for at fair value	0		0
Fair Value, Inputs, Level 2 [Member]
Assets
Fixed maturities, at fair value	26,563,455		25,466,763	[2]
Equity securities, at fair value	308,355	[3]	229,022	[3]
Short-term investments, at fair value	212,327	[4],[5]	359,063	[4],[5]
Total investments available for sale	27,084,137		26,054,848
Cash equivalents	566,444	[6]	1,068,264	[6]
Other investments	727,948	[7]	547,598	[7]
Other assets	116,301	[8],[9]	143,622	[8],[9]
Total assets accounted for at fair value	28,494,830		27,814,332
Liabilities
Financial instruments sold, but not yet purchased	21,575	[10]	20,844	[10]
Other liabilities	26,248	[8],[9]	16,871	[8],[9]
Total liabilities accounted for at fair value	47,823		37,715
Fair Value, Inputs, Level 3 [Member]
Assets
Fixed maturities, at fair value	728,207		723,262	[2]
Equity securities, at fair value	0	[3]	0	[3]
Short-term investments, at fair value	0	[4],[5]	0	[4],[5]
Total investments available for sale	728,207		723,262
Cash equivalents	0	[6]	0	[6]
Other investments	115,659	[7]	113,959	[7]
Other assets	0	[8],[9]	0	[8],[9]
Total assets accounted for at fair value	843,866		837,221
Liabilities
Financial instruments sold, but not yet purchased	0	[10]	0	[10]
Other liabilities	40,630	[8],[9]	42,644	[8],[9]
Total liabilities accounted for at fair value	40,630		42,644
Collateral and Counterparty Netting [Member]
Assets
Fixed maturities, at fair value	0		0	[2]
Equity securities, at fair value	0	[3]	0	[3]
Short-term investments, at fair value	0	[4],[5]	0	[4],[5]
Total investments available for sale	0		0
Cash equivalents	0	[6]	0	[6]
Other investments	0	[7]	0	[7]
Other assets	(77,963)	[8],[9]	(77,888)	[8],[9]
Total assets accounted for at fair value	(77,963)		(77,888)
Liabilities
Financial instruments sold, but not yet purchased	0	[10]	0	[10]
Other liabilities	(512)	[8],[9]	(809)	[8],[9]
Total liabilities accounted for at fair value	$ (512)		$ (809)

[1] Included in the table above 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, at their fair value of $411.2 million and $386.1 million at March 31, 2012 and December 31, 2011, respectively. These securities are reflected in the table based on their call date and have net unrealized losses of $78.1 million and $108.8 million at March 31, 2012 and December 31, 2011, respectively.

[2] The Company invests in covered bonds issued by financial institutions ("Covered Bonds"). Covered Bonds are senior secured debt instruments issued by financial institutions and backed by over-collateralized pools of public sector or mortgage loans. During the three months ended March 31, 2012, Covered Bonds within Total fixed maturities with a fair value of $415.5 million, have been included within Other asset-backed securities to align the Company's classification to market indices. At December 31, 2011 Covered Bonds within Total fixed maturities with a fair value of $353.9 million, have been reclassified from Corporate to Other asset-backed securities to conform to current period presentation.

[3] Included within equity securities are investments in fixed income funds of $98.0 million and $91.6 million at March 31, 2012 and December 31, 2011, respectively.

[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, which are in a gross unrealized loss position, had a fair value of $198.7 million and $266.0 million and an amortized cost of $220.0 million and $297.7 million at March 31, 2012 and December 31, 2011, respectively. 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] Short-term investments consist primarily of Corporate securities and U.S. Government and Government-Related/Supported securities.

[6] Cash equivalents balances subject to fair value measurement include certificates of deposit and money market funds. Operating cash balances are not subject to fair value measurement guidance.

[7] The Other investments balance excludes certain structured transactions including certain investments in project finance transactions, a payment obligation and liquidity financing provided to a structured credit vehicle as a part of a third party medium term note facility. These investments are carried at amortized cost that totaled $316.9 million at March 31, 2012 and $323.7 million at December 31, 2011.

[8] Other assets and other liabilities include derivative instruments.

[9] The derivative balances included in each category above are reported on a gross basis by level with a netting adjustment presented separately in the "Collateral and Counterparty Netting" column. The Company often enters into different types of derivative contracts with a single counterparty and these contracts are covered under a netting agreement. In addition, the Company held net cash collateral related to derivative positions of approximately $77.5 million and $77.1 million at March 31, 2012 and December 31, 2011, respectively. This balance is included within cash and cash equivalents and the corresponding liability to return the collateral has been offset against the derivative positions within the balance sheet as appropriate under the netting agreement. The fair values of the individual derivative contracts are reported gross in their respective levels based on the fair value hierarchy.

[10] Financial instruments sold, but not yet purchased, represent "short sales" and are included within "Payable for investments purchased" on the balance sheet.

[11] 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 $198.7 million and an amortized cost of $220.0 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.

[12] 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 $411.2 million and an amortized cost of $489.3 million at March 31, 2012.

[13] Covered Bonds within Fixed maturities - AFS with an amortized cost of $345.4 million and a fair value of $353.9 million and Covered Bonds within Fixed maturities - HTM with an amortized cost of $8.1 million and a fair value of $7.7 million at December 31, 2011 have been reclassified from Corporate to Other asset-backed securities to align the Company's classification to market indices and conform to current period presentation.

[14] 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 $266.0 million and an amortized cost of $297.7 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.

[15] 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 $386.1 million and an amortized cost of $494.9 million at December 31, 2011.