Quarterly Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Quarterly Financial Data (Unaudited) [Abstract]
Net earned premium	$ 561,186	$ 547,141
Other revenue	73,180	62,382
Total revenue	634,366	609,523
Loss and loss adjustment expense, net	332,697	328,928
Other expense	150,273	163,635
Total expense	482,970	492,563
Earnings before income taxes	151,396	116,960
Income tax expense	45,546	34,376
Net earnings	$ 105,850	$ 82,584
Earnings per share
Basic	$ 1.05	$ 0.80
Diluted	$ 1.05	$ 0.79
Weighted-average shares outstanding
Basic	99,056	102,034
Diluted	99,287	102,193