Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Unrealized Gains (Losses) on Investments arising during the period, Deferred Tax	$ (8,748)	$ 3,730	$ 2,624
Reclassification adjustment for Net Realized (Gains) Losses included in Net Income, Deferred Tax	5,335	(846)	9,185
Non Credit Other-Than-Temporary Impairments arising during the period, Deferred Tax	(16)	27	60
Reclassification Adjustment for Other-Than-Temporary Impairment Credit Losses Recognized in Net Income, Deferred Tax	0	0	(90)
Change in Foreign Currency Translation Gains (Losses), Deferred Tax	$ (2,874)	$ 5,017	$ 351