Share Capital and Capital In Excess of Par Value (Details) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 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			$ (12,959)	$ (19,186)
Purchase and retirement of common shares and other (in shares)			(378)	(510)
Balance at the end of the period	$ 752,391	$ 742,625	$ 752,391	$ 742,625
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	$ 729,673	$ 719,212	$ 696,059	$ 696,535
Issuance of 3% stock dividend			42,243	32,999
Purchase and retirement of common shares and other	(4,068)	(8,509)	(12,697)	(18,831)
Balance at the end of the period	725,605	710,703	725,605	710,703
Class B Common Stock | Common Stock.
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 18,776	$ 18,293	$ 18,254	$ 17,767
Balance at the beginning of the period (in shares)	27,037	26,342	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	$ (9)	$ (29)	$ (34)	$ (35)
Conversion of Class B common shares to common shares (in shares)	(12)	(40)	(48)	(49)
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	26,302	27,025	26,302
Common Stock | Common Stock.
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 27,617	$ 27,374	$ 26,969	$ 26,767
Balance at the beginning of the period (in shares)	39,769	39,418	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	$ 9	$ 29	$ 34	$ 35
Conversion of Class B common shares to common shares (in shares)	12	40	48	49
Purchase and retirement of common shares and other	$ (82)	$ (158)	$ (262)	$ (355)
Purchase and retirement of common shares and other (in shares)	(117)	(225)	(377)	(510)
Balance at the end of the period	$ 27,544	$ 27,245	$ 27,544	$ 27,245
Balance at the end of the period (in shares)	39,664	39,233	39,664	39,233