Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Additional Paid-in Capital	Accumulated other comprehensive income (loss), net of deferred income tax	Retained Earnings	Treasury Shares	Ordinary Shares A	Ordinary Shares A Treasury Shares	Ordinary Shares B
Number at Dec. 31, 2017					29,551	10,102,927		4,133,366
A ordinary shares purchased							45,233
Retirement of shares					0
Ordinary shares issued under share incentive plans						37,381
Ordinary shares issued to directors						16,934
Number at Jun. 30, 2018					74,784	10,157,242		4,133,366
Balance at Dec. 31, 2017		$ 434,730	$ 8,983	$ 275,838	$ (1,159)	$ 1		$ 1
A ordinary shares purchased, at cost							$ (1,813)
Other comprehensive income (loss), net of tax:
Change in unrealized holding gains (losses)			(20,322)
Change in other than temporary impairment losses recognized in other comprehensive income (loss)			(8)
Unrealized foreign currency translation gains (losses)	$ (1,100)		(1,100)
Other comprehensive income (loss), net of tax	(21,430)		(21,430)
Cumulative effect adjustment resulting from adoption of new accounting guidance			(10,028)	10,198
Net income	12,893			12,893
Dividends to shareholders				(7,102)
Share compensation plans		1,305
Balance at Jun. 30, 2018	702,417	436,035	(22,475)	291,827	$ (2,972)	$ 1		$ 1
Number at Mar. 31, 2018					74,784	10,150,130		4,133,366
A ordinary shares purchased							0
Retirement of shares					0
Ordinary shares issued under share incentive plans						(680)
Ordinary shares issued to directors						7,792
Number at Jun. 30, 2018					74,784	10,157,242		4,133,366
Balance at Mar. 31, 2018		435,066	(16,531)	288,187	$ (2,972)	$ 1		$ 1
A ordinary shares purchased, at cost							$ 0
Other comprehensive income (loss), net of tax:
Change in unrealized holding gains (losses)			(5,209)
Change in other than temporary impairment losses recognized in other comprehensive income (loss)			(7)
Unrealized foreign currency translation gains (losses)	(728)		(728)
Other comprehensive income (loss), net of tax	(5,944)		(5,944)
Cumulative effect adjustment resulting from adoption of new accounting guidance			0	0
Net income	7,192			7,192
Dividends to shareholders				(3,552)
Share compensation plans		969
Balance at Jun. 30, 2018	702,417	436,035	(22,475)	291,827	$ (2,972)	$ 1		$ 1
Number at Dec. 31, 2018					76,642	10,171,954		4,133,366
A ordinary shares purchased							27,028
Retirement of shares					6,779
Ordinary shares issued under share incentive plans						36,180
Ordinary shares issued to directors						31,386
Number at Jun. 30, 2019					110,449	10,239,520		4,133,366
Balance at Dec. 31, 2018	629,059	438,182	(21,231)	215,132	$ (3,026)	$ 1		$ 1
A ordinary shares purchased, at cost							$ (947)
Other comprehensive income (loss), net of tax:
Change in unrealized holding gains (losses)			37,644
Change in other than temporary impairment losses recognized in other comprehensive income (loss)			(2)
Unrealized foreign currency translation gains (losses)	131		131
Other comprehensive income (loss), net of tax	37,773		37,773
Cumulative effect adjustment resulting from adoption of new accounting guidance			0	(5)
Net income	34,263			34,263
Dividends to shareholders				(7,156)
Share compensation plans		1,525
Balance at Jun. 30, 2019	694,512	439,707	16,542	242,234	$ (3,973)	$ 1		$ 1
Number at Mar. 31, 2019					110,449	10,223,976		4,133,366
A ordinary shares purchased							0
Retirement of shares					0
Ordinary shares issued under share incentive plans						0
Ordinary shares issued to directors						15,544
Number at Jun. 30, 2019					110,449	10,239,520		4,133,366
Balance at Mar. 31, 2019		438,783	1,669	231,176	$ (3,975)	$ 1		$ 1
A ordinary shares purchased, at cost							$ 2
Other comprehensive income (loss), net of tax:
Change in unrealized holding gains (losses)			14,937
Change in other than temporary impairment losses recognized in other comprehensive income (loss)			(1)
Unrealized foreign currency translation gains (losses)	(63)		(63)
Other comprehensive income (loss), net of tax	14,873		14,873
Cumulative effect adjustment resulting from adoption of new accounting guidance			0	0
Net income	14,663			14,663
Dividends to shareholders				(3,605)
Share compensation plans		924
Balance at Jun. 30, 2019	$ 694,512	$ 439,707	$ 16,542	$ 242,234	$ (3,973)	$ 1		$ 1