Consolidated Statements of Stockholders' Equity (USD $) In Thousands	Total	Common stock [Member]	Additional paid-in capital [Member]	Retained earnings [Member]	Accumulated other comprehensive income [Member]	Treasury stock [Member]
Beginning of year at Dec. 31, 2012		$ 196,388	$ 270,654	$ 1,125,154	$ 54,040	$ (555,644)
Dividend reinvestment plan (shares: 29,949 – 2014; 33,514 - 2013)		67	703
Stock purchase and compensation plans (shares: 547,190 - 2014; 682,661 - 2013)		1,366	10,657
Net income	48,430			48,430
Dividends to stockholders ($0.26 per share - 2014 and 2013)				(14,723)
Other comprehensive income (loss)	(34,762)				(34,762)
Acquisition of treasury stock (shares: 124,819- 2014; 151,113 - 2013)						(3,285)
End of period at Jun. 30, 2013	1,099,045	197,821	282,014	1,158,861	19,278	(558,929)
Beginning of year at Dec. 31, 2013	1,153,928	198,240	288,182	1,202,015	24,851	(559,360)
Dividend reinvestment plan (shares: 29,949 – 2014; 33,514 - 2013)		60	642
Stock purchase and compensation plans (shares: 547,190 - 2014; 682,661 - 2013)		1,095	9,528
Net income	47,315			47,315
Dividends to stockholders ($0.26 per share - 2014 and 2013)				(14,868)
Other comprehensive income (loss)	43,161				43,161
Acquisition of treasury stock (shares: 124,819- 2014; 151,113 - 2013)						(2,786)
End of period at Jun. 30, 2014	$ 1,238,075	$ 199,395	$ 298,352	$ 1,234,462	$ 68,012	$ (562,146)