Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Additional Paid-in Capital	Accumulated other comprehensive income, net of deferred income tax	Retained Earnings	Treasury Shares	Ordinary Shares A	Ordinary Shares A Treasury Shares	Ordinary Shares B
Number at Dec. 31, 2013					3,059,371	16,200,406		12,061,370
Ordinary shares issued under share incentive plans						94,563
A ordinary shares purchased							5,444
Ordinary shares issued to directors						36,608
Number at Dec. 31, 2014					3,064,815	16,331,577		12,061,370
Balance at Dec. 31, 2013		$ 516,653	$ 54,028	$ 403,861	$ (101,265)	$ 2		$ 1
A ordinary shares purchased, at cost							$ (139)
Other comprehensive income (loss):
Change in unrealized holding losses			(30,299)
Change in other than temporary impairment losses recognized in other comprehensive income (loss)			(4)
Unrealized foreign currency translation gains (losses)	$ (341)		(341)
Other comprehensive income (loss), net of tax	(30,644)		(30,644)
Net income	62,856			62,856
Share compensation plans		2,900
Tax benefit on share-based compensation expense	40	37
Balance at Dec. 31, 2014	$ 908,290	519,590	23,384	466,717	$ (101,404)	$ 2		$ 1
Ordinary shares issued under share incentive plans						121,812
A ordinary shares purchased							11,895
Ordinary shares issued to directors						36,321
Elimination of shares indirectly owned by subsidiary							34,085
B ordinary shares converted to A ordinary shares						7,928,004		(7,928,004)
Ordinary shares redeemed	(4,555,061)					(8,260,870)
Ordinary shares issued in connection with American Reliable acquisition						267,702
Number at Dec. 31, 2015					3,110,795	16,424,546		4,133,366
A ordinary shares purchased, at cost							$ (333)
Other comprehensive income (loss):
Change in unrealized holding losses			(19,436)
Change in other than temporary impairment losses recognized in other comprehensive income (loss)			(10)
Unrealized foreign currency translation gains (losses)	$ 140		140
Other comprehensive income (loss), net of tax	(19,306)		(19,306)
Ordinary shares redeemed				(189,770)
Elimination of shares indirectly owned by subsidiary							$ (706)
Net income	41,469			41,469
Share compensation plans		10,272
Tax benefit on share-based compensation expense	50	10
Balance at Dec. 31, 2015	749,926	529,872	4,078	318,416	$ (102,443)	$ 2		$ 1
Ordinary shares issued under share incentive plans						115,711
A ordinary shares purchased							28,099
Ordinary shares issued to directors						35,185
Reduction in treasury shares due to redomestication					(3,138,894)	(3,138,894)
Number at Dec. 31, 2016						13,436,548		4,133,366
A ordinary shares purchased, at cost							$ (805)
Other comprehensive income (loss):
Change in unrealized holding losses			(4,751)
Change in other than temporary impairment losses recognized in other comprehensive income (loss)			(3)
Unrealized foreign currency translation gains (losses)	58		58
Other comprehensive income (loss), net of tax	(4,696)		(4,696)
Net income	49,868			49,868
Reduction in treasury shares due to redomestication		(103,248)			$ 103,248	$ (1)
Share compensation plans		3,532
Tax benefit on share-based compensation expense	100	127
Balance at Dec. 31, 2016	$ 797,951	$ 430,283	$ (618)	$ 368,284		$ 1		$ 1