Schedule I - Consolidated Summary of Investments - other than Investments in Related Parties (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost or Amortized Cost	$ 31,339,033	[1]	$ 30,130,267	[2]
Fair Value	33,283,356		30,871,654
Amount Presented in the Balance Sheet	32,853,308		30,671,525
Fixed Maturities [Member]
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost or Amortized Cost	26,544,806	[1]	25,771,715	[2]
Fair Value	27,846,920		26,190,025
Amount Presented in the Balance Sheet	27,846,920		26,190,025
U.S. Government and Government - Related/Supported [Member]
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost or Amortized Cost	1,906,044	[1]	1,864,354	[2]
Fair Value	2,034,617		1,990,983
Amount Presented in the Balance Sheet	2,034,617		1,990,983
Corporate
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost or Amortized Cost	9,837,962	[1]	9,866,677	[2]
Fair Value	10,482,000		10,108,622
Amount Presented in the Balance Sheet	10,482,000		10,108,622
Residential mortgage-backed securities - Agency ("RMBS - Agency") [Member]
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost or Amortized Cost	5,054,097	[1]	5,189,473	[2]
Fair Value	5,255,493		5,379,406
Amount Presented in the Balance Sheet	5,255,493		5,379,406
Residential mortgage-backed securities - Non-Agency ("RMBS - Non-Agency") [Member]
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost or Amortized Cost	678,469	[1]	851,557	[2]
Fair Value	647,733		641,815
Amount Presented in the Balance Sheet	647,733		641,815
Commercial mortgage-backed securities ("CMBS") [Member]
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost or Amortized Cost	1,010,794	[1]	927,684	[2]
Fair Value	1,077,251		974,835
Amount Presented in the Balance Sheet	1,077,251		974,835
Collateralized debt obligations ("CDO") [Member]
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost or Amortized Cost	784,999	[1]	843,553	[2]
Fair Value	709,816		658,602
Amount Presented in the Balance Sheet	709,816		658,602
Other asset-backed securities [Member]
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost or Amortized Cost	1,426,483	[1]	1,341,309	[2]
Fair Value	1,470,711		1,340,249
Amount Presented in the Balance Sheet	1,470,711		1,340,249
U.S. States and political subdivisions of the States [Member]
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost or Amortized Cost	1,767,669	[1]	1,698,573	[2]
Fair Value	1,911,017		1,797,378
Amount Presented in the Balance Sheet	1,911,017		1,797,378
Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported [Member]
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost or Amortized Cost	4,078,289	[1]	3,188,535	[2]
Fair Value	4,258,282		3,298,135
Amount Presented in the Balance Sheet	4,258,282		3,298,135
Equity securities
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost or Amortized Cost	617,486	[1]	480,685	[2]
Fair Value	649,359		468,197
Amount Presented in the Balance Sheet	649,359		468,197
Short-term investments
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost or Amortized Cost	322,563	[1]	359,378	[2]
Fair Value	322,703		359,063
Amount Presented in the Balance Sheet	322,703		359,063
Fixed maturities, held to maturity
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost or Amortized Cost	2,814,447	[1]	2,668,978	[2]
Fair Value	3,262,804		2,895,688
Amount Presented in the Balance Sheet	2,814,447		2,668,978
Other investments
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost or Amortized Cost	1,039,731	[1]	849,511	[2]
Fair Value	1,201,570		958,681
Amount Presented in the Balance Sheet	$ 1,219,879		$ 985,262

[1] Investments in fixed maturities, short-term investments and held to maturity are shown at amortized cost.

[2] Included within Corporate are certain medium term notes supported primarily by pools of European investment grade credit with varying degrees of leverage.