Reinsurance (Details 5) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Calculation of net reserves, net unearned premium and net deferred policy acquisition costs
Loss and loss adjustment expense payable	$ 3,774,162	$ 3,767,850
Reinsurance recoverable on outstanding losses	(461,886)	(479,026)
Reinsurance recoverable on incurred but not reported losses	(551,034)	(539,021)
Net reserves	2,761,242	2,749,803
Unearned premium	1,085,833	1,069,956
Ceded unearned premium	(257,359)	(256,988)
Net unearned premium	828,474	812,968
Deferred policy acquisition costs	192,199	191,960
Deferred ceding commissions	(72,992)	(74,609)
Net deferred policy acquisition costs	$ 119,207	$ 117,351