Share Capital and Capital In Excess of Par Value (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 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	$ (8,809)	$ (10,519)
Purchase and retirement of common shares and other (in shares)	(260,000)	(285,000)
Balance at the end of the period	$ 751,521	$ 744,660
Treasury stock, shares	93,000	90,000	90,000
Stock dividends (as a percent)	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)
Issuance of 3% stock dividend (in shares)	3,000	2,000
Balance at the end of the period	$ (1,992)	$ (1,992)
Treasury stock, shares	93,000	90,000	90,000	88,000
Capital in Excess of Par Value
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 696,059	$ 696,535
Issuance of 3% stock dividend	42,243	32,999
Purchase and retirement of common shares and other	(8,629)	(10,322)
Balance at the end of the period	729,673	719,212
Class B Common Stock | Common Stock.
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 18,254	$ 17,767
Balance at the beginning of the period (in shares)	26,287,000	25,584,000
Issuance of 3% stock dividend	$ 547	$ 532
Issuance of 3% stock dividend (in shares)	786,000	767,000
Conversion of Class B common shares to common shares	$ (25)	$ (6)
Conversion of Class B common shares to common shares (in shares)	(36,000)	(9,000)
Balance at the end of the period	$ 18,776	$ 18,293
Balance at the end of the period (in shares)	27,037,000	26,342,000
Common Stock | Common Stock.
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 26,969	$ 26,767
Balance at the beginning of the period (in shares)	38,836,000	38,544,000
Issuance of 3% stock dividend	$ 803	$ 798
Issuance of 3% stock dividend (in shares)	1,157,000	1,150,000
Conversion of Class B common shares to common shares	$ 25	$ 6
Conversion of Class B common shares to common shares (in shares)	36,000	9,000
Purchase and retirement of common shares and other	$ (180)	$ (197)
Purchase and retirement of common shares and other (in shares)	(260,000)	(285,000)
Balance at the end of the period	$ 27,617	$ 27,374
Balance at the end of the period (in shares)	39,769,000	39,418,000