Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Unrealized Gains (Losses) on Investments arising during the period, Deferred Tax	$ 2,472	$ (7,907)	$ 11,856	$ (15,433)
Reclassification adjustment for Net Realized (Gains) Losses included in Net Income, Deferred Tax	199	157	943	(215)
Non Credit Other-Than-Temporary Impairments arising during the period, Deferred Tax	4	(10)	16	(16)
Reclassification Adjustment for Other-Than-Temporary Impairment Credit Losses Recognized in Net Income, Deferred Tax	0	0	0	(209)
Change in Foreign Currency Translation Gains (Losses), Deferred Tax	$ 1,361	$ (423)	$ 2,443	$ (280)