Changes in Equity - Schedule of Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 547,177	$ 513,935	$ 485,184	$ 472,939	$ 445,343	$ 512,137	$ 485,184	$ 512,137
Net income	83,837	57,046	50,052	46,275	27,583	57,401	190,935	131,258
Dividends paid	(21,312)	(21,329)	(21,329)			(124,098)
Repurchases of common stock	(8,689)	(2,504)
Conversion of Sub-shares into shares of common stock			0
Other comprehensive income	29	29	28	13	13	14	86	40
Ending balance	$ 601,042	$ 547,177	$ 513,935	$ 519,227	472,939	445,343	$ 601,042	$ 519,227
Cash dividends per Sub-share Certificate (in dollars per share)	$ 2.75	$ 2.75	$ 2.75	$ 0			$ 8.25	$ 16.00
Cumulative effect of accounting change
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance					$ 0	(111)		$ (111)
Ending balance						$ 0
Sub-share Certificates
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)	0	0	7,756,156	7,756,156	7,756,156	7,756,156	7,756,156	7,756,156
Conversion of Sub-shares into shares of common stock (in shares)			(7,756,156)
Ending balance (in shares)	0	0	0	7,756,156	7,756,156	7,756,156	0	7,756,156
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)	7,754,523	7,756,156	0				0
Beginning balance	$ 78	$ 78	$ 0				$ 0
Repurchases of common stock (in shares)	(6,179)	(1,633)
Conversion of Sub-shares into shares of common stock (in shares)			7,756,156
Conversion of Sub-shares into shares of common stock			$ 78
Ending balance (in shares)	7,748,344	7,754,523	7,756,156				7,748,344
Ending balance	$ 78	$ 78	$ 78				$ 78
Treasury Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)	1,633	0	0				0
Beginning balance	$ (2,504)	$ 0	$ 0				$ 0
Repurchases of common stock (in shares)	6,179	1,633					7,812
Repurchases of common stock	$ (8,689)	$ (2,504)
Ending balance (in shares)	7,812	1,633	0				7,812
Ending balance	$ (11,193)	$ (2,504)	$ 0				$ (11,193)
Accum. Other Comp. Loss
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(2,636)	(2,665)	(2,693)	$ (1,434)	$ (1,447)	$ (1,461)	(2,693)	$ (1,461)
Other comprehensive income	29	29	28	13	13	14
Ending balance	(2,607)	(2,636)	(2,665)	(1,421)	(1,434)	(1,447)	(2,607)	(1,421)
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	552,239	516,522	0				0
Net income	83,837	57,046	50,052
Dividends paid	(21,312)	(21,329)	(21,329)
Conversion of Sub-shares into shares of common stock			487,799
Ending balance	614,764	552,239	516,522				614,764
Net Proceeds from All Sources
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	0	0	487,877	474,373	446,790	513,598	487,877	513,598
Net income				46,275	27,583	57,401
Dividends paid						(124,098)
Conversion of Sub-shares into shares of common stock			(487,877)
Ending balance	$ 0	$ 0	$ 0	$ 520,648	474,373	446,790	$ 0	520,648
Net Proceeds from All Sources | Cumulative effect of accounting change
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance					$ 0	(111)		$ (111)
Ending balance						$ 0