Share Capital and Capital In Excess of Par Value (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 750,497
Purchase and retirement of common shares	$ (10,519)	$ (12,498)
Purchase and retirement of common shares (in shares)	(285,000)	(361,000)
Balance at the end of the period	$ 744,660	$ 726,460
Treasury stock, shares	90,000	88,000	88,000
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)	2,000	3,000
Balance at the end of the period	$ (1,992)	$ (1,992)
Treasury stock, shares	90,000	88,000	88,000	85,000
Capital in Excess of Par Value
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 696,535	$ 656,752
Issuance of 3% stock dividend	32,999	58,689
Purchase and retirement of common shares	(10,322)	(12,247)
Balance at the end of the period	719,212	703,194
Class B Common Stock | Common Stock.
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 17,767	$ 17,285
Balance at the beginning of the period (in shares)	25,584,000	24,891,000
Issuance of 3% stock dividend	$ 532	$ 519
Issuance of 3% stock dividend (in shares)	767,000	746,000
Conversion of Class B common shares to common shares	$ (6)
Conversion of Class B common shares to common shares (in shares)	(9,000)
Balance at the end of the period	$ 18,293	$ 17,804
Balance at the end of the period (in shares)	26,342,000	25,637,000
Common Stock | Common Stock.
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 26,767	$ 26,361
Balance at the beginning of the period (in shares)	38,544,000	37,960,000
Issuance of 3% stock dividend	$ 798	$ 781
Issuance of 3% stock dividend (in shares)	1,150,000	1,125,000
Conversion of Class B common shares to common shares	$ 6
Conversion of Class B common shares to common shares (in shares)	9,000
Purchase and retirement of common shares	$ (197)	$ (251)
Purchase and retirement of common shares (in shares)	(285,000)	(361,000)
Balance at the end of the period	$ 27,374	$ 26,891
Balance at the end of the period (in shares)	39,418,000	38,724,000