Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	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	$ 288,182	$ 1,202,015	$ 24,851	$ (559,360)
Dividend reinvestment plan (shares: 38,947 – 2015; 44,322 - 2014)		89	957
Stock purchase and compensation plans (shares: 686,984 - 2015; 588,858 - 2014)		1,178	11,286
Net income	$ 100,477			100,477
Dividends to stockholders ($0.42 per share - 2015; $0.39 per share - 2014)				(22,344)
Other comprehensive (loss) income	24,274				24,274
Acquisition of treasury stock (shares: 139,031 - 2015; 130,573 - 2014)						(2,920)
End of period at Sep. 30, 2014	1,266,925	199,507	300,425	1,280,148	49,125	(562,280)
Beginning of year at Dec. 31, 2014	1,275,586	199,896	305,385	1,313,440	19,788	(562,923)
Dividend reinvestment plan (shares: 38,947 – 2015; 44,322 - 2014)		78	1,014
Stock purchase and compensation plans (shares: 686,984 - 2015; 588,858 - 2014)		1,374	14,588
Net income	120,472			120,472
Dividends to stockholders ($0.42 per share - 2015; $0.39 per share - 2014)				(24,376)
Other comprehensive (loss) income	(25,827)				(25,827)
Acquisition of treasury stock (shares: 139,031 - 2015; 130,573 - 2014)						(3,887)
End of period at Sep. 30, 2015	$ 1,359,022	$ 201,348	$ 320,987	$ 1,409,536	$ (6,039)	$ (566,810)