Consolidated Statements of Stockholders' Equity (USD $)	Total	Common stock [Member]	Additional paid-in capital [Member]	Retained earnings [Member]	Accumulated other comprehensive income (loss) [Member]	Treasury stock [Member]
Beginning of year at Dec. 31, 2011		$ 194,494,000	$ 257,370,000	$ 1,116,319,000	$ 42,294,000	$ (552,149,000)
Dividend reinvestment plan (shares: 58,309 – 2014; 63,349 – 2013; 90,110 – 2012)		180,000	1,419,000
Stock purchase and compensation plans (shares: 769,389 – 2014; 862,662 – 2013; 857,403 – 2012)		1,714,000	11,865,000
Net income	37,963,000			37,963,000
Dividends to stockholders ($0.53 per share – 2014; $0.52 per share – 2013 and 2012)				(29,128,000)
Other comprehensive (loss) income	11,746,000				11,746,000
Acquisition of treasury stock (shares: 154,559 – 2014; 167,846 – 2013; 194,575 – 2012)	(3,495)					(3,495,000)
End of year at Dec. 31, 2012	1,090,592,000	196,388,000	270,654,000	1,125,154,000	54,040,000	(555,644,000)
Dividend reinvestment plan (shares: 58,309 – 2014; 63,349 – 2013; 90,110 – 2012)		127,000	1,396,000
Stock purchase and compensation plans (shares: 769,389 – 2014; 862,662 – 2013; 857,403 – 2012)		1,725,000	16,132,000
Net income	106,418,000			106,418,000
Dividends to stockholders ($0.53 per share – 2014; $0.52 per share – 2013 and 2012)				(29,557,000)
Other comprehensive (loss) income	(29,189,000)				(29,189,000)
Acquisition of treasury stock (shares: 154,559 – 2014; 167,846 – 2013; 194,575 – 2012)	(3,716)					(3,716,000)
End of year at Dec. 31, 2013	1,153,928,000	198,240,000	288,182,000	1,202,015,000	24,851,000	(559,360,000)
Dividend reinvestment plan (shares: 58,309 – 2014; 63,349 – 2013; 90,110 – 2012)		117,000	1,306,000
Stock purchase and compensation plans (shares: 769,389 – 2014; 862,662 – 2013; 857,403 – 2012)		1,539,000	15,897,000
Net income	141,827,000			141,827,000
Dividends to stockholders ($0.53 per share – 2014; $0.52 per share – 2013 and 2012)				(30,402,000)
Other comprehensive (loss) income	(5,063,000)				(5,063,000)
Acquisition of treasury stock (shares: 154,559 – 2014; 167,846 – 2013; 194,575 – 2012)	(3,563)					(3,563,000)
End of year at Dec. 31, 2014	$ 1,275,586,000	$ 199,896,000	$ 305,385,000	$ 1,313,440,000	$ 19,788,000	$ (562,923,000)