Condensed Consolidated Statements of Changes in Capital (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2019
Increase (Decrease) in Stockholders' Equity:
Beginning balance	$ 1,321,316	$ 1,427,509	$ 1,349,210	$ 1,511,913
Common shares issued for compensation and effect of shares reissued to stock purchase plan	678	642	686	675
Share-based compensation	1,153	1,027	2,177	2,044
Net effect of restricted and performance shares issued	(7)	(36)	(264)	(904)
Dividends to shareholders	(2,700)	(2,693)	(5,399)	(19,384)
Other comprehensive income (loss)	11,550	66,040	(22,155)	24,175
Net income (loss)	92,050	(18,099)	99,785	(40,053)
Ending balance	1,424,040	1,474,390	1,424,040	1,474,390	$ 1,511,913
Accounting standards update [extensible list]					us-gaap:AccountingStandardsUpdate201613Member
Cumulative-effect adjustment
Increase (Decrease) in Stockholders' Equity:
Beginning balance				(4,076)
Ending balance					$ (4,076)
Common Stock
Increase (Decrease) in Stockholders' Equity:
Beginning balance	633	632	632	631
Net effect of restricted and performance shares issued			1	1
Ending balance	633	632	633	632	631
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity:
Beginning balance	388,924	384,732	388,150	384,551
Common shares issued for compensation and effect of shares reissued to stock purchase plan	678	642	686	675
Share-based compensation	1,153	1,027	2,177	2,044
Net effect of restricted and performance shares issued	(7)	(36)	(265)	(905)
Ending balance	390,748	386,365	390,748	386,365	384,551
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity:
Beginning balance	41,522	(4,910)	75,227	36,955
Other comprehensive income (loss)	11,550	66,040	(22,155)	24,175
Ending balance	53,072	61,130	53,072	61,130	36,955
Retained Earnings
Increase (Decrease) in Stockholders' Equity:
Beginning balance	1,306,199	1,463,017	1,301,163	1,505,738
Dividends to shareholders	(2,700)	(2,693)	(5,399)	(19,384)
Net income (loss)	92,050	(18,099)	99,785	(40,053)
Ending balance	1,395,549	1,442,225	1,395,549	1,442,225	1,505,738
Retained Earnings | Cumulative-effect adjustment
Increase (Decrease) in Stockholders' Equity:
Beginning balance				(4,076)
Ending balance					(4,076)
Treasury Stock
Increase (Decrease) in Stockholders' Equity:
Beginning balance	(415,962)	(415,962)	(415,962)	(415,962)
Ending balance	$ (415,962)	$ (415,962)	$ (415,962)	$ (415,962)	$ (415,962)