Investments (Net Investment Income) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement [Line Items]
Net investment income earned	$ 35,428	$ 33,375	$ 31,182	$ 30,769	$ 30,108	$ 32,061	$ 32,230	$ 26,917	$ 130,754	$ 121,316	$ 138,708
Fixed Income Securities [Member]
Statement [Line Items]
Net investment income earned									129,306	123,230	126,489
Equity Securities [Member]
Statement [Line Items]
Net investment income earned									7,368	9,161	7,449
Short-term Investments [Member]
Statement [Line Items]
Net investment income earned									686	112	66
Other Investment Portfolio [Member]
Statement [Line Items]
Net investment income earned									2,940	(1,890)	13,580
Investment Expenses [Member]
Statement [Line Items]
Net investment income earned									$ (9,546)	$ (9,297)	$ (8,876)