Share Capital and Capital In Excess of Par Value (Details) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Changes in share capital and capital in excess of par value
Balance at the beginning of the period			$ 759,649
Purchase and retirement of common shares and other			$ (23,505)	$ (27,232)
Purchase and retirement of common shares and other (in shares)			(705)	(726)
Balance at the end of the period	$ 760,738	$ 758,140	$ 760,738	$ 758,140
Treasury stock, shares	93	90	93	90		90
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)			3	2
Balance at the end of the period	$ (1,992)	$ (1,992)	$ (1,992)	$ (1,992)
Treasury stock, shares	93	90	93	90	93	90	90	88
Capital in Excess of Par Value
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 725,605	$ 710,703	$ 696,059	$ 696,535
Issuance of 3% stock dividend			42,243	32,999
Purchase and retirement of common shares and other	(10,319)	(7,897)	(23,016)	(26,728)
Balance at the end of the period	715,286	702,806	715,286	702,806
Class B Common Stock | Common Stock.
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 18,767	$ 18,264	$ 18,254	$ 17,767
Balance at the beginning of the period (in shares)	27,025	26,302	26,287	25,584
Issuance of 3% stock dividend			$ 547	$ 532
Issuance of 3% stock dividend (in shares)			786	767
Conversion of Class B common shares to common shares			$ (34)	$ (35)
Conversion of Class B common shares to common shares (in shares)		(2)	(48)	(51)
Balance at the end of the period	$ 18,767	$ 18,264	$ 18,767	$ 18,264
Balance at the end of the period (in shares)	27,025	26,300	27,025	26,300
Common Stock | Common Stock.
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 27,544	$ 27,245	$ 26,969	$ 26,767
Balance at the beginning of the period (in shares)	39,664	39,233	38,836	38,544
Issuance of 3% stock dividend			$ 803	$ 798
Issuance of 3% stock dividend (in shares)			1,157	1,150
Conversion of Class B common shares to common shares			$ 34	$ 35
Conversion of Class B common shares to common shares (in shares)		2	48	51
Purchase and retirement of common shares and other	$ (227)	$ (149)	$ (489)	$ (504)
Purchase and retirement of common shares and other (in shares)	(328)	(216)	(705)	(726)
Balance at the end of the period	$ 27,317	$ 27,096	$ 27,317	$ 27,096
Balance at the end of the period (in shares)	39,336	39,019	39,336	39,019