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, 2015					3,110,795	16,424,546		4,133,366
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
Balance at Dec. 31, 2015		$ 529,872	$ 4,078	$ 318,416	$ (102,443)	$ 2		$ 1
A ordinary shares purchased, at cost							$ (805)
Other comprehensive income (loss):
Change in unrealized holding gains (losses)			(4,751)
Change in other than temporary impairment losses recognized in other comprehensive income			(3)
Unrealized foreign currency translation gains			58
Other comprehensive income (loss), net of tax			(4,696)
Net income				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		127
Balance at Dec. 31, 2016	$ 797,951	430,283	(618)	368,284		$ 1		1
Ordinary shares issued under share incentive plans						6,473
A ordinary shares purchased							28,965
Ordinary shares issued to directors						6,700
Number at Mar. 31, 2017					28,965	13,449,721
A ordinary shares purchased, at cost							$ (1,137)
Other comprehensive income (loss):
Change in unrealized holding gains (losses)			4,772
Unrealized foreign currency translation gains	178		178
Other comprehensive income (loss), net of tax	4,950		4,950
Net income	12,282			12,282
Share compensation plans		1,121
Balance at Mar. 31, 2017	$ 815,167	$ 431,404	$ 4,332	$ 380,566	$ (1,137)	$ 1		$ 1