Unaudited Quarterly Financial Data (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
REVENUES:
Gross premiums written	$ 555,490	$ 580,893	$ 765,200	$ 837,081	$ 497,052	$ 504,420	$ 646,870	$ 680,929	$ 2,738,664	$ 2,329,271	$ 1,939,521
Premiums ceded	(164,360)	(127,816)	(183,978)	(142,029)	(134,429)	(112,883)	(152,160)	(91,976)	(618,183)	(491,448)	(405,755)	[1]
Net premiums written	391,130	453,077	581,222	695,052	362,623	391,537	494,710	588,953	2,120,481	1,837,823	1,533,766
Change in unearned premiums	133,431	57,696	(73,951)	(231,824)	113,621	49,480	(64,963)	(187,063)	(114,648)	(88,925)	(76,774)
Net premiums earned	524,561	510,773	507,271	463,228	476,244	441,017	429,747	401,890	2,005,833	1,748,898	1,456,992
Net investment income	47,270	39,271	37,635	33,388	38,360	39,121	42,451	47,209	157,564	167,141	195,948
Net realized investment gains	67,599	27,487	(115,198)	79,637	14,379	149,813	8,663	133,581	59,525	306,436	10,077
Total revenues	639,430	577,531	429,708	576,253	528,983	629,951	480,861	582,680	2,222,922	2,222,475	1,764,761
EXPENSES:
Net losses and loss expenses	315,966	276,970	275,128	255,178	414,734	258,948	240,380	225,202	1,123,242	1,139,264	959,156
Acquisition costs	66,257	65,114	64,617	56,685	55,910	51,086	51,588	47,138	252,673	205,722	167,295
General and administrative expenses	100,467	88,553	80,585	82,680	84,404	78,572	73,979	70,366	352,285	307,321	271,656
Amortization of intangible assets	633	633	634	633	633	633	634	633	2,533	2,533	2,978
Interest expense	14,094	14,094	14,188	14,134	13,826	13,822	14,001	13,756	56,510	55,405	54,989
Foreign exchange loss	658	4,353	490	2,518	860	1,023	(1,019)	(81)	8,019	783	3,159
Total expenses	498,075	449,717	435,642	411,828	570,367	404,084	379,563	357,014	1,795,262	1,711,028	1,459,233
Income before income taxes	141,355	127,814	(5,934)	164,425	(41,384)	225,867	101,298	225,666	427,660	511,447	305,528
Income tax expense	3,448	4,971	(4,072)	5,433	(237)	6,220	4,947	7,510	9,780	18,440	30,980
Net income	$ 137,907	$ 122,843	$ (1,862)	$ 158,992	$ (41,147)	$ 219,647	$ 96,351	$ 218,156	$ 417,880	$ 493,007	$ 274,548
PER SHARE DATA
Basic earnings (loss) per share	$ 4.10	$ 3.61	$ (0.05)	$ 4.59	$ (1.17)	$ 6.16	$ 2.66	$ 5.86	$ 12.23	$ 13.67	$ 7.21
Diluted earnings (loss) per share	$ 4.01	$ 3.54	$ (0.05)	$ 4.49	$ (1.17)	$ 6.00	$ 2.59	$ 5.70	$ 11.95	$ 13.30	$ 6.92
Weighted average common shares outstanding	33,604,985	33,991,359	34,422,553	34,613,606	35,097,043	35,652,768	36,288,596	37,205,166	34,154,905	36,057,145	38,093,351
Weighted average common shares and common share equivalents outstanding	34,376,604	34,728,193	34,422,553	35,431,843	35,097,043	36,616,734	37,189,722	38,284,635	34,955,278	37,069,885	39,667,905

[1] Includes the impact of the commutation of prior year variable-rated excess-of-loss reinsurance contracts, which decreased ceded premiums written and earned by $12,393 and increased prior year ceded losses and loss expenses by $11,529.