Quarterly Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Quarterly Financial Data (Unaudited) [Abstract]
Net earned premium	$ 561,356	$ 565,331	$ 1,122,542	$ 1,112,472
Other revenue	68,075	66,957	141,255	129,339
Total revenue	629,431	632,288	1,263,797	1,241,811
Loss and loss adjustment expense, net	339,474	336,825	672,171	665,753
Other expense	165,126	161,144	315,399	324,779
Total expense	504,600	497,969	987,570	990,532
Earnings before income taxes	124,831	134,319	276,227	251,279
Income tax expense	36,669	40,826	82,215	75,202
Net earnings	$ 88,162	$ 93,493	$ 194,012	$ 176,077
Earnings per share
Basic	$ 0.88	$ 0.92	$ 1.93	$ 1.71
Diluted	$ 0.87	$ 0.92	$ 1.92	$ 1.71
Weighted-average shares outstanding
Basic	98,870	99,563	98,965	100,802
Diluted	99,121	99,851	99,210	100,983