Unpaid Losses and Loss Expenses and Policy Benefits for Life and Annuity Contracts (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Liability For Unpaid Claims And Claims Adjustment Expense Net Abstract
Gross liability at beginning of year				$ 11,273,091,000				$ 10,666,604,000	$ 11,273,091,000	$ 10,666,604,000	$ 10,811,483,000
Reinsurance recoverables at beginning of year				353,105,000				348,747,000	353,105,000	348,747,000	336,352,000
Net liability at beginning of year				10,919,986,000				10,317,857,000	10,919,986,000	10,317,857,000	10,475,131,000
Net incurred losses related to:
Current Year									2,785,694,000	4,252,766,000	3,137,874,000
Prior Years									(628,065,000)	(530,457,000)	(477,883,000)
Total incurred losses									2,157,629,000	3,722,309,000	2,659,991,000
Change in Paris Re Reserve Agreement									(86,163,000)	(61,383,000)	(66,783,000)
Net paid losses related to:
Current Year									237,783,000	930,407,000	311,253,000
Prior Years									2,467,279,000	2,060,152,000	2,267,765,000
Total paid losses									2,705,062,000	2,990,559,000	2,579,018,000
Effects of foreign exchange rate changes	131,651,000				(68,238,000)				131,651,000	(68,238,000)	(171,464,000)
Net liability at end of year	10,418,041,000				10,919,986,000				10,418,041,000	10,919,986,000	10,317,857,000
Reinsurance recoverable at end of year	291,330,000				353,105,000				291,330,000	353,105,000	348,747,000
Gross liability at end of year	10,709,371,000				11,273,091,000				10,709,371,000	11,273,091,000	10,666,604,000
Liability for unpaid losses and loss expenses by type
Case reserves	4,872,591,000				5,187,761,000				4,872,591,000	5,187,761,000
Additional case reserves (ACRs)	354,382,000				495,593,000				354,382,000	495,593,000
IBNR reserves	5,482,398,000				5,589,737,000				5,482,398,000	5,589,737,000
Unpaid losses and loss expenses	10,709,371,000				11,273,091,000				10,709,371,000	11,273,091,000	10,666,604,000
Paris Re Reserve Agreement
Guaranteed reserves related to Paris Re	857,000,000				1,012,000,000				857,000,000	1,012,000,000
Amounts payable to Colisee Re under reserve agreements	12,000,000				183,000,000				12,000,000	183,000,000
Losses And Loss Expenses And Life Policy Benefits [Line Items]
Losses and loss expenses and life policy benefits	800,900,000	721,100,000	706,100,000	576,500,000	1,069,200,000	881,700,000	814,500,000	1,607,200,000	2,804,610,000	4,372,570,000	3,283,618,000
[LifeMember]
Losses And Loss Expenses And Life Policy Benefits [Line Items]
Losses and loss expenses and life policy benefits									646,981,000	650,261,000	623,627,000
[NonLifeMember]
Losses And Loss Expenses And Life Policy Benefits [Line Items]
Losses and loss expenses and life policy benefits									$ 2,157,629,000	$ 3,722,309,000	$ 2,659,991,000