Condensed Consolidated Statements of Changes in Capital (Unaudited) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019	Jan. 01, 2019
Increase (Decrease) in Stockholders' Equity:
Beginning balance		$ 1,511,913	$ 1,523,002
Common shares issued for compensation and effect of shares reissued to stock purchase plan		33	176
Share-based compensation		1,017	1,237
Net effect of restricted and performance shares issued		(868)	(2,631)
Dividends to shareholders		(16,691)	(16,660)
Other comprehensive income (loss)		(41,865)	25,566
Net income		(21,954)	31,650
Ending balance		1,427,509	1,561,896
Accounting Standards Update 2018-07
Increase (Decrease) in Stockholders' Equity:
Cumulative-effect adjustment adoption of ASU	[1]			$ (444)
Common Stock
Increase (Decrease) in Stockholders' Equity:
Beginning balance		631	630
Net effect of restricted and performance shares issued		1	1
Ending balance		632	631
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity:
Beginning balance		384,551	384,713
Common shares issued for compensation and effect of shares reissued to stock purchase plan		33	176
Share-based compensation		1,017	1,237
Net effect of restricted and performance shares issued		(869)	(2,632)
Ending balance		384,732	383,494
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity:
Beginning balance		36,955	(16,911)
Other comprehensive income (loss)		(41,865)	25,566
Ending balance		(4,910)	8,655
Retained Earnings
Increase (Decrease) in Stockholders' Equity:
Beginning balance		1,505,738	1,571,847
Dividends to shareholders		(16,691)	(16,660)
Net income		(21,954)	31,650
Ending balance		1,463,017	1,586,393
Retained Earnings | Accounting Standards Update 2018-07
Increase (Decrease) in Stockholders' Equity:
Cumulative-effect adjustment adoption of ASU	[1]			$ (444)
Treasury Stock
Increase (Decrease) in Stockholders' Equity:
Beginning balance		(415,962)	(417,277)
Ending balance		$ (415,962)	$ (417,277)

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