Investments (Details) - Analysis of net investment income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Investments [Abstract]
Fixed maturities, short term investments and cash equivalents	$ 1,046,255	$ 1,178,038	$ 1,245,185
Equity securities and other investments	29,807	17,804	20,693
Funds withheld	12,090	12,240	12,738
Total gross investment income	1,088,152	1,208,082	1,278,616
Investment expenses	(75,804)	(70,313)	(80,578)
Net investment income	$ 1,012,348	$ 1,137,769	$ 1,198,038