Reinsurance (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Premiums written
Assumed									$ 5,569,706,000	$ 4,718,235,000	$ 4,633,054,000
Ceded									173,180,000	145,375,000	146,725,000
Net	1,186,000,000	1,265,000,000	1,309,000,000	1,636,000,000	920,000,000	1,043,000,000	1,136,000,000	1,473,000,000	5,396,526,000	4,572,860,000	4,486,329,000
Premiums earned
Assumed									5,373,866,000	4,640,949,000	4,789,293,000
Ceded									175,656,000	155,010,000	141,539,000
Net	1,421,000,000	1,421,000,000	1,209,000,000	1,147,000,000	1,168,000,000	1,237,000,000	1,091,000,000	990,000,000	5,198,210,000	4,485,939,000	4,647,754,000
Loss and Loss Expenses and Life Policy Benefits
Assumed									3,207,860,000	2,838,117,000	4,456,094,000
Ceded									50,052,000	33,507,000	83,524,000
Net	879,000,000	751,000,000	867,000,000	661,000,000	801,000,000	721,000,000	706,000,000	576,000,000	3,157,808,000	2,804,610,000	4,372,570,000
Allowance For Reinsurance Recoverable [Member]
Allowance for uncollectible reinsurance recoverable [Line Items]
Allowance for uncollectible reinsurance recoverable	$ 15,000,000				$ 13,000,000				$ 15,000,000	$ 13,000,000