Unpaid Losses and Loss Expenses and Policy Benefits for Life and Annuity Contracts (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Liability For Unpaid Claims And Claims Adjustment Expense Net Abstract
Gross liability at beginning of year				$ 10,666,604,000				$ 10,811,483,000	$ 10,666,604,000	$ 10,811,483,000	$ 7,510,666,000
Reinsurance recoverables at beginning of year				348,747,000				336,352,000	348,747,000	336,352,000	125,215,000
Net liability at beginning of year				10,317,857,000				10,475,131,000	10,317,857,000	10,475,131,000	7,385,451,000
Net liability acquired related to Paris Re	0				0				0	0	3,176,255,000
Net incurred losses related to:
Current Year									4,252,766,000	3,137,874,000	2,340,768,000
Prior Years									(530,457,000)	(477,883,000)	(485,809,000)
Total incurred losses									3,722,309,000	2,659,991,000	1,854,959,000
Change in Paris Re Reserve Agreement									(61,383,000)	(66,783,000)	(32,027,000)
Net paid losses related to:
Current Year									930,407,000	311,253,000	327,080,000
Prior Years									2,060,152,000	2,267,765,000	1,716,798,000
Total paid losses									2,990,559,000	2,579,018,000	2,043,878,000
Effects of foreign exchange rate changes	(68,238,000)				(171,464,000)				(68,238,000)	(171,464,000)	134,371,000
Net liability at end of year	10,919,986,000				10,317,857,000				10,919,986,000	10,317,857,000	10,475,131,000
Reinsurance recoverable at end of year	353,105,000				348,747,000				353,105,000	348,747,000	336,352,000
Gross liability at end of year	11,273,091,000				10,666,604,000				11,273,091,000	10,666,604,000	10,811,483,000
Liability for unpaid losses and loss expenses by type
Case reserves	5,187,761,000				4,652,281,000				5,187,761,000	4,652,281,000
Additional case reserves (ACRs)	495,593,000				326,721,000				495,593,000	326,721,000
IBNR reserves	5,589,737,000				5,687,602,000				5,589,737,000	5,687,602,000
Unpaid losses and loss expenses	11,273,091,000				10,666,604,000				11,273,091,000	10,666,604,000	10,811,483,000
Paris Re Reserve Agreement
Guaranteed reserves related to Paris Re	1,012,000,000				1,239,000,000				1,012,000,000	1,239,000,000
Amounts payable to Colisee Re under reserve agreements	183,000,000				128,000,000				183,000,000	128,000,000
Losses And Loss Expenses And Life Policy Benefits [Line Items]
Losses and loss expenses and life policy benefits	1,069,200,000	881,700,000	814,500,000	1,607,200,000	817,800,000	748,900,000	704,600,000	1,012,400,000	4,372,570,000	3,283,618,000	2,295,296,000
[LifeMember]
Losses And Loss Expenses And Life Policy Benefits [Line Items]
Losses and loss expenses and life policy benefits									650,261,000	623,627,000	440,337,000
[NonLifeMember]
Losses And Loss Expenses And Life Policy Benefits [Line Items]
Losses and loss expenses and life policy benefits									$ 3,722,309,000	$ 2,659,991,000	$ 1,854,959,000