Share Capital and Capital In Excess of Par Value (Details) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 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	$ (7,423)	$ (8,809)
Purchase and retirement of common shares and other (in shares)	(234)	(260)
Balance at the end of the period	$ 758,586	$ 751,521
Balance at the end of the period (in shares)	96	93
Stock dividends (as a percent)	3.00%	3.00%
Common Stock.
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 27,134	$ 26,969
Balance at the beginning of the period (in shares)	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		$ 25
Conversion of Class B common shares to common shares (in shares)		36
Purchase and retirement of common shares and other	$ (163)	$ (180)
Purchase and retirement of common shares and other (in shares)	(234)	(260)
Balance at the end of the period	$ 27,779	$ 27,617
Balance at the end of the period (in shares)	40,002	39,769
Treasury Stock
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ (1,992)	$ (1,992)
Balance at the beginning of the period (in shares)	93	90
Issuance of 3% stock dividend (in shares)	3	3
Balance at the end of the period	$ (1,992)	$ (1,992)
Balance at the end of the period (in shares)	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	$ 706,930	$ 696,059
Issuance of 3% stock dividend	32,495	42,243
Purchase and retirement of common shares and other	(7,260)	(8,629)
Balance at the end of the period	732,165	729,673
Class B Common Stock | Common Stock.
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 18,758	$ 18,254
Balance at the beginning of the period (in shares)	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		$ (25)
Conversion of Class B common shares to common shares (in shares)		(36)
Balance at the end of the period	$ 19,320	$ 18,776
Balance at the end of the period (in shares)	27,821	27,037