Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Components of net investment income
Total investment income	$ 341,733	$ 338,293	$ 321,866
Investment expenses	(9,133)	(8,478)	(8,256)
Net investment income	332,600	329,815	313,610
Equity Securities [Member]
Components of net investment income
Total investment income	4,355	4,849	3,698
Fixed Maturities [Member]
Components of net investment income
Total investment income	326,207	317,756	304,024
Short Term And Other Investments [Member]
Components of net investment income
Total investment income	9,187	8,459	8,242
Other Invested Asset [Member]
Components of net investment income
Total investment income	$ 1,984	$ 7,229	$ 5,902