Investments (Net Investment Income) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement [Line Items]
Net investment income earned	$ 42,587	$ 40,446	$ 41,430	$ 37,419	$ 35,428	$ 33,375	$ 31,182	$ 30,769	$ 161,882	$ 130,754	$ 121,316
Fixed Income Securities [Member]
Statement [Line Items]
Net investment income earned									153,230	129,306	123,230
Equity Securities [Member]
Statement [Line Items]
Net investment income earned									6,442	7,368	9,161
Short-term Investments [Member]
Statement [Line Items]
Net investment income earned									1,526	686	112
Other Investment Portfolio [Member]
Statement [Line Items]
Net investment income earned									12,871	2,940	(1,890)
Investment Expenses [Member]
Statement [Line Items]
Net investment income earned									$ (12,187)	$ (9,546)	$ (9,297)