Share Capital and Capital In Excess of Par Value (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Changes in share capital and capital in excess of par value
Balance at the beginning of the period			$ 750,497
Purchase and retirement of common shares			$ (19,186)	$ (15,803)
Purchase and retirement of common shares (in shares)			(510,000)	(472,000)
Balance at the end of the period	$ 742,625	$ 726,031	$ 742,625	$ 726,031
Treasury stock, shares	90,000	88,000	90,000	88,000		88,000
Stock dividends (as a percent)	3.00%	3.00%	3.00%	3.00%
Treasury Stock
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ (1,992)	$ (1,992)	$ (1,992)	$ (1,992)
Issuance of 3% stock dividend (in shares)			2,000	3,000
Balance at the end of the period	$ (1,992)	$ (1,992)	$ (1,992)	$ (1,992)
Treasury stock, shares	90,000	88,000	90,000	88,000	90,000	88,000	88,000	85,000
Capital in Excess of Par Value
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 719,212	$ 703,194	$ 696,535	$ 656,752
Issuance of 3% stock dividend			32,999	58,689
Purchase and retirement of common shares	(8,509)	(3,229)	(18,831)	(15,476)
Balance at the end of the period	710,703	699,965	710,703	699,965
Class B Common Stock | Common Stock.
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 18,293	$ 17,804	$ 17,767	$ 17,285
Balance at the beginning of the period (in shares)	26,342,000	25,637,000	25,584,000	24,891,000
Issuance of 3% stock dividend			$ 532	$ 518
Issuance of 3% stock dividend (in shares)			767,000	746,000
Conversion of Class B common shares to common shares	$ (29)	$ (23)	$ (35)	$ (22)
Conversion of Class B common shares to common shares (in shares)	(40,000)	(32,000)	(49,000)	(32,000)
Balance at the end of the period	$ 18,264	$ 17,781	$ 18,264	$ 17,781
Balance at the end of the period (in shares)	26,302,000	25,605,000	26,302,000	25,605,000
Common Stock | Common Stock.
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 27,374	$ 26,891	$ 26,767	$ 26,361
Balance at the beginning of the period (in shares)	39,418,000	38,724,000	38,544,000	37,960,000
Issuance of 3% stock dividend			$ 798	$ 781
Issuance of 3% stock dividend (in shares)			1,150,000	1,125,000
Conversion of Class B common shares to common shares	$ 29	$ 23	$ 35	$ 22
Conversion of Class B common shares to common shares (in shares)	40,000	32,000	49,000	32,000
Purchase and retirement of common shares	$ (158)	$ (77)	$ (355)	$ (327)
Purchase and retirement of common shares (in shares)	(225,000)	(111,000)	(510,000)	(472,000)
Balance at the end of the period	$ 27,245	$ 26,837	$ 27,245	$ 26,837
Balance at the end of the period (in shares)	39,233,000	38,645,000	39,233,000	38,645,000