Investments (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Components of net investment income
Total investment income	$ 321,866	$ 313,857	$ 295,358
Investment expenses	(8,256)	(7,854)	(7,047)
Net investment income	313,610	306,003	288,311
Fixed maturities [Member]
Components of net investment income
Total investment income	304,024	297,042	285,782
Equity securities [Member]
Components of net investment income
Total investment income	3,698	2,814	1,685
Short-term and other investments [Member]
Components of net investment income
Total investment income	8,242	8,109	7,891
Other invested asset [Member]
Components of net investment income
Total investment income	$ 5,902	$ 5,892	$ 0