Investments, Investment Income By Category (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
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income									$ 124,378	$ 113,882	$ 106,736
Investment expenses									(3,507)	(3,216)	(2,995)
Net investment income	$ 31,276	$ 30,402	$ 29,716	$ 29,477	$ 28,094	$ 27,515	$ 27,248	$ 27,809	120,871	110,666	103,741
Fixed income
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income									122,105	112,513	105,882
Equity securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income									206	182	208
Cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income									1,447	754	191
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income									$ 620	$ 433	$ 455