Unaudited Quarterly Financial Data (Details) (USD $) In Thousands, except Share data, 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
REVENUES:
Gross premiums written	$ 565,743	$ 707,884	$ 760,405	$ 901,393	$ 555,490	$ 580,893	$ 765,200	$ 837,081	$ 2,935,425	$ 2,738,664	$ 2,329,271
Premiums ceded	(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
Change in unearned premiums	145,694	(27,005)	(16,677)	(241,329)	133,431	57,696	(73,951)	(231,824)	(139,317)	(114,648)	(88,925)
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
Net investment income	49,050	43,412	36,793	47,619	47,270	39,271	37,635	33,388	176,874	157,564	167,141
Net realized investment gains	(15,329)	(35,136)	85,217	54,205	67,599	27,487	(115,198)	79,637	88,957	59,525	306,436
Other income	1,032	1,032	0	0					2,064	0	0
Total revenues	608,242	551,045	659,257	632,109	639,430	577,531	429,708	576,253	2,450,653	2,222,922	2,222,475
EXPENSES:
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
Acquisition costs	80,718	72,403	74,279	67,722	66,257	65,114	64,617	56,685	295,122	252,673	205,722
General and administrative expenses	100,914	88,294	96,188	80,340	100,467	88,553	80,585	82,680	365,736	352,285	307,321
Other expense	2,003	6,575	0	0					8,578	0	0
Amortization of intangible assets	633	633	634	633	633	633	634	633	2,533	2,533	2,533
Interest expense	14,304	14,325	14,592	14,534	14,094	14,094	14,188	14,134	57,755	56,510	55,405
Foreign exchange loss	(21)	278	651	49	658	4,353	490	2,518	957	8,019	783
Total expenses	471,510	518,598	501,199	438,564	498,075	449,717	435,642	411,828	1,929,871	1,795,262	1,711,028
Income before income taxes	136,732	32,447	158,058	193,545	141,355	127,814	(5,934)	164,425	520,782	427,660	511,447
Income tax expense	6,223	1,532	6,195	16,573	3,448	4,971	(4,072)	5,433	30,523	9,780	18,440
Net income	$ 130,509	$ 30,915	$ 151,863	$ 176,972	$ 137,907	$ 122,843	$ (1,862)	$ 158,992	$ 490,259	$ 417,880	$ 493,007
PER SHARE DATA
Basic earnings (loss) per share	$ 1.35	$ 0.32	$ 1.55	$ 1.78	$ 1.37	$ 1.20	$ (0.02)	$ 1.53	$ 5.03	$ 4.08	$ 4.56
Diluted earnings (loss) per share	$ 1.33	$ 0.31	$ 1.52	$ 1.74	$ 1.34	$ 1.18	$ (0.02)	$ 1.50	$ 4.92	$ 3.98	$ 4.43
Weighted average common shares outstanding	96,386,796	96,458,231	97,809,639	99,545,187	100,814,955	101,974,077	103,267,659	103,840,818	97,538,319	102,464,715	108,171,435
Weighted average common shares and common share equivalents outstanding	98,394,432	98,444,238	99,724,802	101,584,662	103,129,812	104,184,579	103,267,659	106,295,529	99,591,773	104,865,834	111,209,655