Investments, Investment Income By Category (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income									$ 159,608	$ 171,347	$ 145,006
Investment expenses									(5,212)	(4,302)	(3,675)
Net investment income	$ 36,118	$ 37,252	$ 39,679	$ 41,347	$ 41,322	$ 42,715	$ 42,423	$ 40,585	154,396	167,045	141,331
Fixed income securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income									157,065	165,523	140,539
Equity securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income									620	406	228
Cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income									1,648	4,444	3,423
Other investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income									$ 275	$ 974	$ 816