Consolidated Statements of Changes in Capital - USD ($) $ in Thousands		3 Months Ended				12 Months Ended
		Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Jan. 01, 2018	Jan. 01, 2017
Increase (Decrease) in Stockholders' Equity:
Beginning Balance			$ 1,523,002		$ 1,594,795	$ 1,523,002	$ 1,594,795	$ 1,798,702
Common shares issued for compensation and effect of shares reissued to stock purchase plan						350	1,044	2,880
Share-based compensation						3,512	5,258	10,615
Net effect of restricted and performance shares issued						(2,708)	(3,934)	(5,437)
Dividends to shareholders						(66,669)	(94,314)	(316,890)
Other comprehensive income (loss)						53,866	(35,238)	(2,488)
Net income		$ (59,375)	31,650	$ (24,450)	11,856	1,004	47,057	107,264
Ending Balance		1,511,913		1,523,002		1,511,913	1,523,002	1,594,795
Accounting Standards Update 2016-09
Increase (Decrease) in Stockholders' Equity:
Cumulative-effect adjustment ASU adoption											$ 149
Accounting Standards Update 2016-01
Increase (Decrease) in Stockholders' Equity:
Cumulative-effect adjustment ASU adoption										$ 8,334
Accounting Standards Update 2018-07
Increase (Decrease) in Stockholders' Equity:
Cumulative-effect adjustment ASU adoption	[1]								$ (444)
Common Stock
Increase (Decrease) in Stockholders' Equity:
Beginning Balance			630		628	630	628	627
Net effect of restricted and performance shares issued						1	2	1
Ending Balance		631		630		631	630	628
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity:
Beginning Balance			384,713		383,077	384,713	383,077	376,518
Common shares issued for compensation and effect of shares reissued to stock purchase plan						(965)	314	957
Share-based compensation						3,512	5,258	10,615
Net effect of restricted and performance shares issued						(2,709)	(3,936)	(5,438)
Ending Balance		384,551		384,713		384,551	384,713	383,077
Additional Paid-in Capital | Accounting Standards Update 2016-09
Increase (Decrease) in Stockholders' Equity:
Cumulative-effect adjustment ASU adoption											425
Additional Paid-in Capital | Accounting Standards Update 2018-07
Increase (Decrease) in Stockholders' Equity:
Cumulative-effect adjustment ASU adoption	[1]								0
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity:
Beginning Balance			(16,911)		14,911	(16,911)	14,911	17,399
Other comprehensive income (loss)						53,866	(35,238)	(2,488)
Ending Balance		36,955		(16,911)		36,955	(16,911)	14,911
Accumulated Other Comprehensive Income (Loss) | Accounting Standards Update 2018-02
Increase (Decrease) in Stockholders' Equity:
Cumulative-effect adjustment ASU adoption										3,416
Retained Earnings
Increase (Decrease) in Stockholders' Equity:
Beginning Balance			1,571,847		1,614,186	1,571,847	1,614,186	1,824,088
Dividends to shareholders						(66,669)	(94,314)	(316,890)
Net income						1,004	47,057	107,264
Ending Balance		1,505,738		1,571,847		1,505,738	1,571,847	1,614,186
Retained Earnings | Accounting Standards Update 2016-09
Increase (Decrease) in Stockholders' Equity:
Cumulative-effect adjustment ASU adoption											$ (276)
Retained Earnings | Accounting Standards Update 2016-01
Increase (Decrease) in Stockholders' Equity:
Cumulative-effect adjustment ASU adoption										8,334
Retained Earnings | Accounting Standards Update 2018-02
Increase (Decrease) in Stockholders' Equity:
Cumulative-effect adjustment ASU adoption										$ (3,416)
Retained Earnings | Accounting Standards Update 2018-07
Increase (Decrease) in Stockholders' Equity:
Cumulative-effect adjustment ASU adoption	[1]								$ (444)
Treasury Stock
Increase (Decrease) in Stockholders' Equity:
Beginning Balance			$ (417,277)		$ (418,007)	(417,277)	(418,007)	(419,930)
Common shares issued for compensation and effect of shares reissued to stock purchase plan						1,315	730	1,923
Ending Balance		$ (415,962)		$ (417,277)		$ (415,962)	$ (417,277)	$ (418,007)

[1] See Note 1 of the Notes to Consolidated Financial Statements for discussion of accounting guidance adopted during the year.