Consolidated Statements of Changes in Equity and Comprehensive Income - USD ($) $ in Thousands		Total	Common Stock [Member]	Premium on Common Stock [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Loss [Member]	Retained Earnings [Member]
Balance at Dec. 31, 2012		$ 736,214	$ 79,134	$ 306,422	$ (364)	$ (31,105)	$ 382,127
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income		81,593					81,593
Other Comprehensive (Loss) Income, Net of Tax	[1]	10,345				10,345
Common Stock Issued or Granted Under Stock Plans		56,527	2,654	53,881	(8)
Cash Dividends Declared - Common Stock ($1.02 per share, $0.96 per share, and $0.90 per share for 2015, 2014, and 2013, respectively)		(57,679)					(57,679)
Balance at Dec. 31, 2013		$ 827,000	81,788	360,303	(372)	(20,760)	406,041
Additional Statement Information [Abstract]
Combined statutory tax rate		41.00%
Net Income		$ 97,046					97,046
Other Comprehensive (Loss) Income, Net of Tax	[1]	(9,498)				(9,498)
Common Stock Issued or Granted Under Stock Plans		81,753	3,630	78,081	42
Cash Dividends Declared - Common Stock ($1.02 per share, $0.96 per share, and $0.90 per share for 2015, 2014, and 2013, respectively)		(63,869)					(63,869)
Balance at Dec. 31, 2014		$ 932,432	85,418	438,384	(330)	(30,258)	439,218
Additional Statement Information [Abstract]
Combined statutory tax rate		40.00%
Net Income		$ 105,107					105,107
Other Comprehensive (Loss) Income, Net of Tax	[1]	5,759				5,759
Common Stock Issued or Granted Under Stock Plans		64,399	3,289	61,076	34
Cash Dividends Declared - Common Stock ($1.02 per share, $0.96 per share, and $0.90 per share for 2015, 2014, and 2013, respectively)		(70,158)					(70,158)
Balance at Dec. 31, 2015		$ 1,037,539	$ 88,707	$ 499,460	$ (296)	$ (24,499)	$ 474,167
Additional Statement Information [Abstract]
Combined statutory tax rate		40.00%

[1] For 2015 and 2014, determined using a combined average statutory tax rate of 40%. For 2013, determined using a combined statutory tax rate of 41%.