Reinsurance (Details 5) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011
Calculation of net reserves, net unearned premium and net deferred policy acquisition costs
Loss and loss adjustment expense payable	$ 4,021,847		$ 3,767,850
Reinsurance recoverable on outstanding losses	(456,364)		(479,026)
Reinsurance recoverable on incurred but not reported losses	(709,451)		(539,021)
Net reserves	2,856,032	2,811,021	2,749,803	2,735,013	2,748,995	2,683,483
Unearned premium	1,165,580		1,069,956
Ceded unearned premium	(301,891)		(256,988)
Net unearned premium	863,689		812,968
Deferred policy acquisition costs	204,740		191,960
Deferred ceding commissions	(86,804)		(74,609)
Net deferred policy acquisition costs	$ 117,936		$ 117,351