Condensed Consolidated Statements of Changes in Capital (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended			12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020		Sep. 30, 2019	Dec. 31, 2019		Dec. 31, 2018
Increase (Decrease) in Stockholders' Equity:
Beginning balance	$ 1,474,390	$ 1,581,556	$ 1,511,913		$ 1,523,002	$ 1,523,002
Common shares issued for compensation and effect of shares reissued to stock purchase plan	8	20	683		832
Share-based compensation	1,017	(11)	3,061		2,309
Net effect of restricted and performance shares issued	(4)	(28)	(908)		(2,672)
Dividends to shareholders	(2,694)	(16,677)	(22,078)		(49,992)
Other comprehensive income (loss)	7,155	7,405	31,330		56,045
Net income (loss)	(149,979)	17,193	(190,032)		60,378
Ending balance	1,329,893	1,589,458	$ 1,329,893		1,589,458	$ 1,511,913		$ 1,523,002
Accounting standards update [extensible list]			us-gaap:AccountingStandardsUpdate201613Member			us-gaap:AccountingStandardsUpdate201613Member		us-gaap:AccountingStandardsUpdate201807Member
Common Stock
Increase (Decrease) in Stockholders' Equity:
Beginning balance	632	631	$ 631		630	$ 630
Net effect of restricted and performance shares issued			1		1
Ending balance	632	631	632		631	631		$ 630
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity:
Beginning balance	386,365	385,200	384,551		384,713	384,713
Common shares issued for compensation and effect of shares reissued to stock purchase plan	8	20	683		832
Share-based compensation	1,017	(11)	3,061		2,309
Net effect of restricted and performance shares issued	(4)	(28)	(909)		(2,673)
Ending balance	387,386	385,181	387,386		385,181	384,551		384,713
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity:
Beginning balance	61,130	31,729	36,955		(16,911)	(16,911)
Other comprehensive income (loss)	7,155	7,405	31,330		56,045
Ending balance	68,285	39,134	68,285		39,134	36,955		(16,911)
Retained Earnings
Increase (Decrease) in Stockholders' Equity:
Beginning balance	1,442,225	1,581,273	1,505,738		1,571,847	1,571,847
Dividends to shareholders	(2,694)	(16,677)	(22,078)		(49,992)
Net income (loss)	(149,979)	17,193	(190,032)		60,378
Ending balance	1,289,552	1,581,789	1,289,552		1,581,789	1,505,738		1,571,847
Treasury Stock
Increase (Decrease) in Stockholders' Equity:
Beginning balance	(415,962)	(417,277)	(415,962)		(417,277)	(417,277)
Common shares issued for compensation and effect of shares reissued to stock purchase plan	0
Ending balance	$ (415,962)	$ (417,277)	(415,962)		(417,277)	(415,962)		(417,277)
Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity:
Beginning balance			(4,076)	[1]	(444)	(444)
Ending balance						(4,076)	[1]	(444)
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings
Increase (Decrease) in Stockholders' Equity:
Beginning balance			$ (4,076)	[1]	$ (444)	(444)
Ending balance						$ (4,076)	[1]	$ (444)

[1] See Note 1 for discussion of accounting guidance adopted during the period.