Reinsurance (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
Premiums written
Assumed Premiums Written									$ 4,718,235,000	$ 4,633,054,000	$ 4,885,266,000
Ceded									145,375,000	146,725,000	180,150,000
Net	920,300,000	1,043,200,000	1,136,000,000	1,473,300,000	879,900,000	1,079,600,000	1,056,500,000	1,470,400,000	4,572,860,000	4,486,329,000	4,705,116,000
Premiums Earned Net Consolidated [Abstract]
Assumed									4,640,949,000	4,789,293,000	4,956,897,000
Ceded									155,010,000	141,539,000	180,426,000
Net premiums earned	1,168,100,000	1,237,100,000	1,090,900,000	989,800,000	1,181,400,000	1,294,300,000	1,107,500,000	1,064,600,000	4,485,939,000	4,647,754,000	4,776,471,000
Loss and Loss Expenses and Life Policy Benefits
Assumed									2,838,117,000	4,456,094,000	3,399,157,000
Ceded									33,507,000	83,524,000	115,539,000
Net	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
[AllowanceForReinsuranceRecoverableMember]
Allowance for uncollectible reinsurance recoverable Line Items
Allowance for uncollectible reinsurance recoverable	$ 13,500,000				$ 12,500,000				$ 13,500,000	$ 12,500,000