Consolidated Statements of Changes in Capital - USD ($) $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Increase (Decrease) in Stockholders' Equity:
Beginning Balance		$ 1,958,354		$ 2,157,944	$ 1,958,354	$ 2,157,944	$ 2,394,414
Common shares reacquired					(2,106)	(169,793)	(222,360)
Common shares issued for compensation and effect of shares reissued to stock purchase plan					3,432	3,629	4,200
Share-based compensation					12,455	9,166	10,056
Net effect of restricted and performance shares issued and stock options exercised					(3,030)	(4,574)	(3,010)
Dividends to shareholders					(315,028)	(119,866)	(220,464)
Other comprehensive income (loss)					(6,456)	(34,349)	(1,457)
Net income	$ 54,848	19,317	$ 34,948	37,814	151,081	116,197	196,565
Ending Balance	1,798,702		1,958,354		1,798,702	1,958,354	2,157,944
Common Stock
Increase (Decrease) in Stockholders' Equity:
Beginning Balance		625		623	625	623	621
Net effect of restricted and performance shares issued and stock options exercised					2	2	2
Ending Balance	627		625		627	625	623
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity:
Beginning Balance		365,399		359,577	365,399	359,577	349,894
Common shares issued for compensation and effect of shares reissued to stock purchase plan					1,696	1,232	2,639
Share-based compensation					12,455	9,166	10,056
Net effect of restricted and performance shares issued and stock options exercised					(3,032)	(4,576)	(3,012)
Ending Balance	376,518		365,399		376,518	365,399	359,577
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity:
Beginning Balance		23,855		58,204	23,855	58,204	59,661
Other comprehensive income (loss)					(6,456)	(34,349)	(1,457)
Ending Balance	17,399		23,855		17,399	23,855	58,204
Retained Earnings
Increase (Decrease) in Stockholders' Equity:
Beginning Balance		1,988,035		1,991,704	1,988,035	1,991,704	2,015,603
Dividends to shareholders					(315,028)	(119,866)	(220,464)
Net income					151,081	116,197	196,565
Ending Balance	1,824,088		1,988,035		1,824,088	1,988,035	1,991,704
Treasury Stock
Increase (Decrease) in Stockholders' Equity:
Beginning Balance		$ (419,560)		$ (252,164)	(419,560)	(252,164)	(31,365)
Common shares reacquired					(2,106)	(169,793)	(222,360)
Common shares issued for compensation and effect of shares reissued to stock purchase plan					1,736	2,397	1,561
Ending Balance	$ (419,930)		$ (419,560)		$ (419,930)	$ (419,560)	$ (252,164)