Reinsurance (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Reinsurance [Abstract]
Written premiums, Direct	$ 815,087	$ 823,378	$ 824,366
Written premiums, Assumed	3,471,087	3,377,341	3,304,589
Written premiums, Ceded	(177,275)	(255,141)	(199,194)
Net written premiums	4,108,899	3,945,578	3,929,761
Premiums earned, Direct	871,598	823,811	808,793
Premiums earned, Assumed	3,454,622	3,340,977	3,255,849
Premiums earned, Ceded	(224,873)	(230,163)	(170,544)
Net premiums earned	4,101,347	3,934,625	3,894,098
Incurred losses and LAE, Direct	744,017	708,212	655,263
Incurred losses and LAE, Assumed	3,099,787	2,392,758	1,829,371
Incurred losses and LAE, Ceded	(117,600)	(155,258)	(110,576)
Net incurred losses and LAE	$ 3,726,204	$ 2,945,712	$ 2,374,058