Ceded Reinsurance (Effects of Reinsurance) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reinsurance Disclosures [Abstract]
Direct premiums written									$ 1,996,831	$ 1,804,910	$ 1,569,021
Assumed premiums written									938,594	933,754	760,250
Ceded premiums written	(137,948)	(139,142)	(206,481)	(129,779)	(164,360)	(127,816)	(183,978)	(142,029)	(613,350)	(618,183)	(491,448)
Net premiums written	427,795	568,742	553,924	771,614	391,130	453,077	581,222	695,052	2,322,075	2,120,481	1,837,823
Direct premiums earned									1,835,186	1,664,343	1,446,086
Assumed premiums earned									941,182	896,088	743,575
Ceded premiums earned									(593,610)	(554,598)	(440,763)
Net premiums earned	573,489	541,737	537,247	530,285	524,561	510,773	507,271	463,228	2,182,758	2,005,833	1,748,898
Direct losses and loss expenses									1,031,284	936,084	877,961
Assumed losses and loss expenses									464,728	430,627	513,871
Ceded losses and loss expenses									(296,822)	(243,469)	(252,568)
Net losses and loss expenses	$ 272,959	$ 336,090	$ 314,855	$ 275,286	$ 315,966	$ 276,970	$ 275,128	$ 255,178	$ 1,199,190	$ 1,123,242	$ 1,139,264