Consolidated Statements of Stockholders' Equity - USD ($)	Total	Common Stock [Member]	Additional paid-in capital [Member]	Retained earnings [Member]	Accumulated other comprehensive (loss) income [Member]	Treasury stock [Member]
Beginning of year at Dec. 31, 2013		$ 198,240,000	$ 288,182,000	$ 1,202,015,000	$ 24,851,000	$ (559,360,000)
Dividend reinvestment plan (shares: 38,741 – 2016; 50,013 – 2015; 58,309 – 2014)		117,000	1,306,000
Stock purchase and compensation plans (shares: 720,323 – 2016; 863,426 – 2015; 769,389 – 2014)		1,539,000	15,897,000
Net income	$ 141,827,000			141,827,000
Dividends to stockholders ($0.61 per share – 2016; $0.57 per share – 2015; $0.53 per share – 2014)				(30,402,000)
Other comprehensive loss	(5,063,000)				(5,063,000)
Acquisition of treasury stock (shares: 152,595 – 2016; 147,461 – 2015; 154,559 – 2014)						(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)
Dividend reinvestment plan (shares: 38,741 – 2016; 50,013 – 2015; 58,309 – 2014)		100,000	1,374,000
Stock purchase and compensation plans (shares: 720,323 – 2016; 863,426 – 2015; 769,389 – 2014)		1,727,000	19,897,000
Net income	165,861,000			165,861,000
Dividends to stockholders ($0.61 per share – 2016; $0.57 per share – 2015; $0.53 per share – 2014)				(33,109,000)
Other comprehensive loss	(29,213,000)				(29,213,000)
Acquisition of treasury stock (shares: 152,595 – 2016; 147,461 – 2015; 154,559 – 2014)						(4,182,000)
End of year at Dec. 31, 2015	1,398,041,000	201,723,000	326,656,000	1,446,192,000	(9,425,000)	(567,105,000)
Dividend reinvestment plan (shares: 38,741 – 2016; 50,013 – 2015; 58,309 – 2014)		77,000	1,389,000
Stock purchase and compensation plans (shares: 720,323 – 2016; 863,426 – 2015; 769,389 – 2014)		1,441,000	19,250,000
Net income	158,495,000			158,495,000
Dividends to stockholders ($0.61 per share – 2016; $0.57 per share – 2015; $0.53 per share – 2014)				(35,806,000)
Other comprehensive loss	(6,525,000)				(6,525,000)
Acquisition of treasury stock (shares: 152,595 – 2016; 147,461 – 2015; 154,559 – 2014)						(4,992,000)
End of year at Dec. 31, 2016	$ 1,531,370,000	$ 203,241,000	$ 347,295,000	$ 1,568,881,000	$ (15,950,000)	$ (572,097,000)