Investments, Investment Income By Category (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
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income									$ 113,882	$ 106,736	$ 90,554
Investment expenses									(3,216)	(2,995)	(2,907)
Net investment income	$ 28,094	$ 27,515	$ 27,248	$ 27,809	$ 27,926	$ 25,939	$ 25,756	$ 24,120	110,666	103,741	87,647
Fixed income
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income									112,513	105,882	89,437
Equity securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income									182	208	227
Cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income									754	191	179
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income									$ 433	$ 455	$ 711