Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Earnings Per Share [Abstract]
Net income	$ 269,980	$ 176,049	$ 318,728
Basic earnings per share:
Weighted average number of Sub-share Certificates outstanding - basic (in shares)	7,752,027	7,756,156	7,756,437
Basic earnings per share (in dollars per share)	$ 34.83	$ 22.70	$ 41.09
Diluted earnings per share:
Weighted average number of Sub-share Certificates outstanding - basic (in shares)	7,752,027	7,756,156	7,756,437
Stock-based incentive plan	27	0	0
Weighted average number of Sub-share Certificates outstanding - diluted (in shares)	7,752,054	7,756,156	7,756,437
Diluted earnings per share (in dollars per share)	$ 34.83	$ 22.70	$ 41.09