Acquisitions (Unaudited Pro Forma Consolidated Financial Information) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Acquisitions [Abstract]
Earned premiums	$ 919,723		$ 862,534		$ 2,717,732		$ 2,592,818
Operating revenues	1,234,380		1,146,994		3,629,659		3,365,605
Net income to shareholders	$ 90,910		$ 60,090		$ 298,970		$ 265,664
U.S. GAAP combined ratio	95.00%	[1]	102.00%	[1]	95.00%	[1]	96.00%	[1]
Basic net income per share	$ 6.49		$ 4.29		$ 21.33		$ 18.96
Diluted net income per share	$ 6.47		$ 4.27		$ 21.24		$ 18.90
Weighted average common shares outstanding, basic	14,001		14,015		14,014		14,012
Weighted average common shares outstanding, diluted	14,057		14,067		14,075		14,060

[1] The U.S. GAAP combined ratio is a measure of underwriting performance and represents the relationship of incurred losses, loss adjustment expenses and underwriting, acquisition and insurance expenses to earned premiums.