Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	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, 2014		$ 199,896	$ 305,385	$ 1,313,440	$ 19,788	$ (562,923)
Dividend reinvestment plan (shares: 20,808 – 2016; 26,843 - 2015)		54	677
Stock purchase and compensation plans (shares: 569,034 - 2016; 611,076 - 2015)		1,222	11,233
Net income	$ 73,476			73,476
Dividends to stockholders ($0.30 per share - 2016; $0.28 per share - 2015)				(16,235)
Other comprehensive income (loss)	(32,117)				(32,117)
Acquisition of treasury stock (shares: 138,007 - 2016; 129,352 - 2015)						(3,583)
End of period at Jun. 30, 2015	1,310,313	201,172	317,295	1,370,681	(12,329)	(566,506)
Beginning of year at Dec. 31, 2015	1,398,041	201,723	326,656	1,446,192	(9,425)	(567,105)
Dividend reinvestment plan (shares: 20,808 – 2016; 26,843 - 2015)		42	696
Stock purchase and compensation plans (shares: 569,034 - 2016; 611,076 - 2015)		1,138	12,757
Net income	80,633			80,633
Dividends to stockholders ($0.30 per share - 2016; $0.28 per share - 2015)				(17,583)
Other comprehensive income (loss)	81,432				81,432
Acquisition of treasury stock (shares: 138,007 - 2016; 129,352 - 2015)						(4,419)
End of period at Jun. 30, 2016	$ 1,552,737	$ 202,903	$ 340,109	$ 1,509,242	$ 72,007	$ (571,524)