Schedule III Supplementary Insurance Information (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Premium Revenue	$ 7,906,596,000	$ 7,335,687,000	$ 6,659,680,000
Supplementary Insurance Information, Net Investment Income	1,436,206,000	1,281,197,000	1,238,660,000
Supplementary Insurance Information, Benefits, Claims, Losses and Settlement Expense	(7,045,914,000)	(6,541,577,000)	(5,857,137,000)
Supplementary Insurance Information, Amortization of Deferred Policy Acquisition Costs	(1,003,057,000)	(691,123,000)	(928,105,000)
Supplementary Insurance Information, Other Operating Expense	(872,717,000)	(833,267,000)	(670,307,000)
Reinsurance Assumed [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Deferred Policy Acquisition Costs	3,677,803,000	3,605,611,000
Supplementary Insurance Information, Liability for Future Policy Benefits, Losses, Claims and Loss Expense Reserves	24,726,358,000	18,298,354,000
Supplementary Insurance Information, Other Policy Claims and Benefits Payable	3,160,250,000	2,841,373,000
Reinsurance Ceded [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Deferred Policy Acquisition Costs	(58,529,000)	(61,686,000)
Supplementary Insurance Information, Liability for Future Policy Benefits, Losses, Claims and Loss Expense Reserves	(581,088,000)	(538,526,000)
Supplementary Insurance Information, Other Policy Claims and Benefits Payable	(166,468,000)	(155,034,000)
US Segment Reporting Information [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Premium Revenue	4,322,875,000	3,992,678,000	3,797,081,000
Supplementary Insurance Information, Net Investment Income	1,034,088,000	858,536,000	865,798,000
Supplementary Insurance Information, Benefits, Claims, Losses and Settlement Expense	(4,123,965,000)	(3,787,801,000)	(3,539,577,000)
Supplementary Insurance Information, Amortization of Deferred Policy Acquisition Costs	(654,609,000)	(345,426,000)	(631,945,000)
Supplementary Insurance Information, Other Operating Expense	(307,650,000)	(356,191,000)	(268,704,000)
US Segment Reporting Information [Member] | Reinsurance Assumed [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Deferred Policy Acquisition Costs	2,598,662,000	2,563,919,000
Supplementary Insurance Information, Liability for Future Policy Benefits, Losses, Claims and Loss Expense Reserves	18,818,559,000	13,236,919,000
Supplementary Insurance Information, Other Policy Claims and Benefits Payable	1,212,107,000	1,094,172,000
US Segment Reporting Information [Member] | Reinsurance Ceded [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Deferred Policy Acquisition Costs	(32,453,000)	(35,292,000)
Supplementary Insurance Information, Liability for Future Policy Benefits, Losses, Claims and Loss Expense Reserves	(231,383,000)	(221,872,000)
Supplementary Insurance Information, Other Policy Claims and Benefits Payable	(88,649,000)	(71,323,000)
Canada Segment Reporting Information [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Premium Revenue	915,764,000	835,298,000	797,206,000
Supplementary Insurance Information, Net Investment Income	190,337,000	188,304,000	169,136,000
Supplementary Insurance Information, Benefits, Claims, Losses and Settlement Expense	(706,744,000)	(673,105,000)	(656,358,000)
Supplementary Insurance Information, Amortization of Deferred Policy Acquisition Costs	(167,614,000)	(146,680,000)	(144,680,000)
Supplementary Insurance Information, Other Operating Expense	(78,935,000)	(71,293,000)	(57,394,000)
Canada Segment Reporting Information [Member] | Reinsurance Assumed [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Deferred Policy Acquisition Costs	271,656,000	250,262,000
Supplementary Insurance Information, Liability for Future Policy Benefits, Losses, Claims and Loss Expense Reserves	3,007,476,000	2,762,175,000
Supplementary Insurance Information, Other Policy Claims and Benefits Payable	210,334,000	192,187,000
Canada Segment Reporting Information [Member] | Reinsurance Ceded [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Deferred Policy Acquisition Costs	(510,000)	(524,000)
Supplementary Insurance Information, Liability for Future Policy Benefits, Losses, Claims and Loss Expense Reserves	(236,586,000)	(215,176,000)
Supplementary Insurance Information, Other Policy Claims and Benefits Payable	(20,460,000)	(20,364,000)
Europe And South Africa Segment Reporting Information [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Premium Revenue	1,308,462,000	1,194,477,000	918,513,000
Supplementary Insurance Information, Net Investment Income	45,576,000	44,351,000	37,039,000
Supplementary Insurance Information, Benefits, Claims, Losses and Settlement Expense	(1,134,219,000)	(1,001,921,000)	(734,392,000)
Supplementary Insurance Information, Amortization of Deferred Policy Acquisition Costs	(50,174,000)	(35,833,000)	(35,154,000)
Supplementary Insurance Information, Other Operating Expense	(113,951,000)	(129,003,000)	(118,564,000)
Europe And South Africa Segment Reporting Information [Member] | Reinsurance Assumed [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Deferred Policy Acquisition Costs	332,780,000	347,112,000
Supplementary Insurance Information, Liability for Future Policy Benefits, Losses, Claims and Loss Expense Reserves	887,404,000	718,167,000
Supplementary Insurance Information, Other Policy Claims and Benefits Payable	712,392,000	573,019,000
Europe And South Africa Segment Reporting Information [Member] | Reinsurance Ceded [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Deferred Policy Acquisition Costs	(10,599,000)	(12,092,000)
Supplementary Insurance Information, Liability for Future Policy Benefits, Losses, Claims and Loss Expense Reserves	(36,097,000)	(32,945,000)
Supplementary Insurance Information, Other Policy Claims and Benefits Payable	(20,570,000)	(22,166,000)
Asia Pacific Segment Reporting Information [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Premium Revenue	1,350,330,000	1,304,490,000	1,139,065,000
Supplementary Insurance Information, Net Investment Income	83,387,000	84,837,000	71,827,000
Supplementary Insurance Information, Benefits, Claims, Losses and Settlement Expense	(1,081,010,000)	(1,077,982,000)	(926,383,000)
Supplementary Insurance Information, Amortization of Deferred Policy Acquisition Costs	(130,660,000)	(162,926,000)	(115,496,000)
Supplementary Insurance Information, Other Operating Expense	(238,497,000)	(147,272,000)	(127,703,000)
Asia Pacific Segment Reporting Information [Member] | Reinsurance Assumed [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Deferred Policy Acquisition Costs	474,705,000	444,318,000
Supplementary Insurance Information, Liability for Future Policy Benefits, Losses, Claims and Loss Expense Reserves	1,712,908,000	1,567,628,000
Supplementary Insurance Information, Other Policy Claims and Benefits Payable	1,015,119,000	970,756,000
Asia Pacific Segment Reporting Information [Member] | Reinsurance Ceded [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Deferred Policy Acquisition Costs	(14,967,000)	(13,778,000)
Supplementary Insurance Information, Liability for Future Policy Benefits, Losses, Claims and Loss Expense Reserves	(77,015,000)	(68,533,000)
Supplementary Insurance Information, Other Policy Claims and Benefits Payable	(36,534,000)	(40,964,000)
Corporate And Other Segment Reporting Information [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Premium Revenue	9,165,000	8,744,000	7,815,000
Supplementary Insurance Information, Net Investment Income	82,818,000	105,169,000	94,860,000
Supplementary Insurance Information, Benefits, Claims, Losses and Settlement Expense	24,000	(768,000)	(427,000)
Supplementary Insurance Information, Amortization of Deferred Policy Acquisition Costs	0	(258,000)	(830,000)
Supplementary Insurance Information, Other Operating Expense	(133,684,000)	(129,508,000)	(97,942,000)
Corporate And Other Segment Reporting Information [Member] | Reinsurance Assumed [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Deferred Policy Acquisition Costs	0	0
Supplementary Insurance Information, Liability for Future Policy Benefits, Losses, Claims and Loss Expense Reserves	300,011,000	13,465,000
Supplementary Insurance Information, Other Policy Claims and Benefits Payable	10,298,000	11,239,000
Corporate And Other Segment Reporting Information [Member] | Reinsurance Ceded [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Deferred Policy Acquisition Costs	0	0
Supplementary Insurance Information, Liability for Future Policy Benefits, Losses, Claims and Loss Expense Reserves	(7,000)	0
Supplementary Insurance Information, Other Policy Claims and Benefits Payable	$ (255,000)	$ (217,000)