Condensed Consolidated Statements of Changes in Capital (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Increase (Decrease) in Stockholders' Equity:
Beginning Balance			$ 1,798,702	$ 1,958,354
Common shares reacquired			0	(2,106)
Common shares issued for compensation and effect of shares reissued to stock purchase plan			1,878	2,195
Share-based compensation			6,092	5,813
Net effect of restricted and performance shares issued and stock options exercised			(5,323)	(2,981)
Dividends to shareholders			(33,040)	(32,913)
Other comprehensive income (loss)			8,665	37,434
Net income	$ 19,518	$ 43,081	60,973	62,399
Ending Balance	1,838,096	2,028,195	1,838,096	2,028,195
Common Stock [Member]
Increase (Decrease) in Stockholders' Equity:
Beginning Balance			627	625
Common shares issued for compensation and effect of shares reissued to stock purchase plan				1
Net effect of restricted and performance shares issued and stock options exercised			1	1
Ending Balance	628	627	628	627
Additional Paid-in Capital [Member]
Increase (Decrease) in Stockholders' Equity:
Beginning Balance			376,518	365,399
Common shares issued for compensation and effect of shares reissued to stock purchase plan			1,876	2,194
Share-based compensation			6,092	5,813
Net effect of restricted and performance shares issued and stock options exercised			(5,324)	(2,982)
Ending Balance	379,587	370,424	379,587	370,424
Accumulated Other Comprehensive Income (Loss) [Member]
Increase (Decrease) in Stockholders' Equity:
Beginning Balance			17,399	23,855
Other comprehensive income (loss)			8,665	37,434
Ending Balance	26,064	61,289	26,064	61,289
Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity:
Beginning Balance			1,824,088	1,988,035
Dividends to shareholders			(33,040)	(32,913)
Net income			60,973	62,399
Ending Balance	1,851,745	2,017,521	1,851,745	2,017,521
Treasury Stock [Member]
Increase (Decrease) in Stockholders' Equity:
Beginning Balance			419,930	(419,560)
Common shares reacquired			0	(2,106)
Common shares issued for compensation and effect of shares reissued to stock purchase plan			2	0
Ending Balance	$ (419,928)	$ (421,666)	$ (419,928)	$ (421,666)