Share Capital and Capital In Excess of Par Value (Details) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Changes in share capital and capital in excess of par value
Balance at the beginning of the period			$ 763,100
Balance at the beginning of the period (in shares)			93
Purchase and retirement of common shares and other			$ (17,181)	$ (12,959)
Purchase and retirement of common shares and other (in shares)			(536)	(378)
Balance at the end of the period	$ 752,586	$ 752,391	$ 752,586	$ 752,391
Balance at the end of the period (in shares)	96	93	96	93
Stock dividends (as a percent)	3.00%	3.00%	3.00%	3.00%
Common Stock.
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 27,779	$ 27,617	$ 27,134	$ 26,969
Balance at the beginning of the period (in shares)	40,002	39,769	39,073	38,836
Issuance of 3% stock dividend			$ 808	$ 803
Issuance of 3% stock dividend (in shares)			1,163	1,157
Conversion of Class B common shares to common shares		$ 9		$ 34
Conversion of Class B common shares to common shares (in shares)		12		48
Purchase and retirement of common shares and other	$ (210)	$ (82)	$ (373)	$ (262)
Purchase and retirement of common shares and other (in shares)	(302)	(117)	(536)	(377)
Balance at the end of the period	$ 27,569	$ 27,544	$ 27,569	$ 27,544
Balance at the end of the period (in shares)	39,700	39,664	39,700	39,664
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)
Balance at the beginning of the period (in shares)	96	93	93	90
Issuance of 3% stock dividend (in shares)			3	3
Balance at the end of the period	$ (1,992)	$ (1,992)	$ (1,992)	$ (1,992)
Balance at the end of the period (in shares)	96	93	96	93
Capital in Excess of Par Value
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 732,165	$ 729,673	$ 706,930	$ 696,059
Issuance of 3% stock dividend			32,495	42,243
Purchase and retirement of common shares and other	(9,548)	(4,068)	(16,808)	(12,697)
Balance at the end of the period	722,617	725,605	722,617	725,605
Class B Common Stock | Common Stock.
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 19,320	$ 18,776	$ 18,758	$ 18,254
Balance at the beginning of the period (in shares)	27,821	27,037	27,012	26,287
Issuance of 3% stock dividend			$ 562	$ 547
Issuance of 3% stock dividend (in shares)			809	786
Conversion of Class B common shares to common shares		$ (9)		$ (34)
Conversion of Class B common shares to common shares (in shares)		(12)		(48)
Balance at the end of the period	$ 19,320	$ 18,767	$ 19,320	$ 18,767
Balance at the end of the period (in shares)	27,821	27,025	27,821	27,025