Condensed Consolidated Statements of Changes in Capital (Unaudited) - USD ($)		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Increase (Decrease) in Stockholders' Equity:
Beginning Balance		$ 1,594,795,000	$ 1,798,702,000
Common shares issued for compensation and effect of shares reissued to stock purchase plan		120,000	940,000
Share-based compensation		902,000	3,346,000
Net effect of restricted and performance shares issued		(3,849,000)	(5,314,000)
Dividends to shareholders		(16,616,000)	(16,516,000)
Other comprehensive income (loss)		(26,373,000)	2,924,000
Net income		11,856,000	41,455,000
Ending Balance		1,569,169,000	1,825,686,000
Accounting Standards Update 2016-01
Increase (Decrease) in Stockholders' Equity:
Cumulative-effect adjustment adoption of ASU	[1]			$ 8,334,000
Accounting Standards Update 2016-09
Increase (Decrease) in Stockholders' Equity:
Cumulative-effect adjustment adoption of ASU	[1]				$ 149,000
Common Stock
Increase (Decrease) in Stockholders' Equity:
Beginning Balance		628,000	627,000
Net effect of restricted and performance shares issued		2,000	1,000
Ending Balance		630,000	628,000
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity:
Beginning Balance		383,077,000	376,518,000
Common shares issued for compensation and effect of shares reissued to stock purchase plan		122,000	938,000
Share-based compensation		902,000	3,346,000
Net effect of restricted and performance shares issued		(3,851,000)	(5,315,000)
Ending Balance		380,250,000	375,912,000
Additional Paid-in Capital | Accounting Standards Update 2016-09
Increase (Decrease) in Stockholders' Equity:
Cumulative-effect adjustment adoption of ASU	[1]				425,000
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity:
Beginning Balance		14,911,000	17,399,000
Other comprehensive income (loss)		(26,373,000)	2,924,000
Ending Balance		(8,046,000)	20,323,000
Accumulated Other Comprehensive Income (Loss) | Accounting Standards Update 2018-02
Increase (Decrease) in Stockholders' Equity:
Cumulative-effect adjustment adoption of ASU	[1]			3,416,000
Retained Earnings
Increase (Decrease) in Stockholders' Equity:
Beginning Balance		1,614,186,000	1,824,088,000
Dividends to shareholders		(16,616,000)	(16,516,000)
Net income		11,856,000	41,455,000
Ending Balance		1,614,344,000	1,848,751,000
Retained Earnings | Accounting Standards Update 2016-01
Increase (Decrease) in Stockholders' Equity:
Cumulative-effect adjustment adoption of ASU	[1]			8,334,000
Retained Earnings | Accounting Standards Update 2018-02
Increase (Decrease) in Stockholders' Equity:
Cumulative-effect adjustment adoption of ASU	[1]			$ (3,416,000)
Retained Earnings | Accounting Standards Update 2016-09
Increase (Decrease) in Stockholders' Equity:
Cumulative-effect adjustment adoption of ASU	[1]				$ (276,000)
Treasury Stock
Increase (Decrease) in Stockholders' Equity:
Beginning Balance		(418,007,000)	(419,930,000)
Common shares issued for compensation and effect of shares reissued to stock purchase plan		(2,000)	2,000
Ending Balance		$ (418,009,000)	$ (419,928,000)

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