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						23,983
Number at Sep. 30, 2018					74,784	10,164,291		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)			(21,229)
Change in other than temporary impairment losses recognized in other comprehensive income (loss)			(1)
Unrealized foreign currency translation gains (losses)	$ (1,554)		(1,554)
Other comprehensive income (loss), net of tax	(22,784)		(22,784)
Cumulative effect adjustment resulting from adoption of new accounting guidance			(10,028)	10,198
Net income	16,621			16,621
Dividends to shareholders	(10,656)			(10,656)
Share compensation plans		2,394
Balance at Sep. 30, 2018	702,326	437,124	(23,829)	292,001	$ (2,972)	$ 1		$ 1
Number at Jun. 30, 2018					74,784	10,157,242		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						7,049
Number at Sep. 30, 2018					74,784	10,164,291		4,133,366
Balance at Jun. 30, 2018		436,035	(22,475)	291,827	$ (2,972)	$ 1		$ 1
A ordinary shares purchased, at cost							$ 0
Other comprehensive income (loss), net of tax:
Change in unrealized holding gains (losses)			(907)
Change in other than temporary impairment losses recognized in other comprehensive income (loss)			7
Unrealized foreign currency translation gains (losses)	(454)		(454)
Other comprehensive income (loss), net of tax	(1,354)		(1,354)
Cumulative effect adjustment resulting from adoption of new accounting guidance			0	0
Net income	3,728			3,728
Dividends to shareholders				(3,554)
Share compensation plans		1,089
Balance at Sep. 30, 2018	702,326	437,124	(23,829)	292,001	$ (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						50,661
Number at Sep. 30, 2019					110,449	10,258,795		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)			46,218
Change in other than temporary impairment losses recognized in other comprehensive income (loss)			(4)
Unrealized foreign currency translation gains (losses)	331		331
Other comprehensive income (loss), net of tax	46,545		46,545
Cumulative effect adjustment resulting from adoption of new accounting guidance			0	(5)
Net income	40,984			40,984
Dividends to shareholders	(10,765)			(10,765)
Share compensation plans		2,513
Balance at Sep. 30, 2019	707,384	440,695	25,314	245,346	$ (3,973)	$ 1		$ 1
Number at Jun. 30, 2019					110,449	10,239,520		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						19,275
Number at Sep. 30, 2019					110,449	10,258,795		4,133,366
Balance at Jun. 30, 2019		439,707	16,542	242,234	$ (3,973)	$ 1		$ 1
A ordinary shares purchased, at cost							$ 0
Other comprehensive income (loss), net of tax:
Change in unrealized holding gains (losses)			8,574
Change in other than temporary impairment losses recognized in other comprehensive income (loss)			(2)
Unrealized foreign currency translation gains (losses)	200		200
Other comprehensive income (loss), net of tax	8,772		8,772
Cumulative effect adjustment resulting from adoption of new accounting guidance			0	0
Net income	6,721			6,721
Dividends to shareholders				(3,609)
Share compensation plans		988
Balance at Sep. 30, 2019	$ 707,384	$ 440,695	$ 25,314	$ 245,346	$ (3,973)	$ 1		$ 1