Investments, Investment Income By Category (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income									$ 171,347	$ 145,006	$ 124,378
Investment expenses									(4,302)	(3,675)	(3,507)
Net investment income	$ 41,322	$ 42,715	$ 42,423	$ 40,585	$ 38,328	$ 36,380	$ 34,502	$ 32,121	167,045	141,331	120,871
Fixed income securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income									165,523	140,539	122,105
Equity securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income									406	228	206
Cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income									4,444	3,423	1,447
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income									$ 974	$ 816	$ 620