Consolidated Statement of Shareholders' Equity (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Balance	$ 733,033	$ 714,488	$ 693,101	$ 660,105
Comprehensive income	58,364	62,702	72,031
Dividends paid on common stock	(48,007)	(46,690)	(44,652)
Tax expense from employee stock option plan	(149)	(26)	(125)
Stock-based compensation	1,291	1,769	554
Issuance of common stock	7,046	3,632	5,188
Common stock expense		0
Common Stock [Member]
Balance	356,571	348,383	342,978	337,361
Comprehensive income	0	0	0
Dividends paid on common stock	0	0	0
Tax expense from employee stock option plan	(149)	(26)	(125)
Stock-based compensation	1,291	1,769	554
Issuance of common stock	7,046	3,632	5,188
Common stock expense		30
Accumulated Other Comprehensive Income (Loss) [Member]
Balance	(9,291)	(7,800)	(6,604)	(5,968)
Comprehensive income	(1,491)	(1,196)	(636)
Dividends paid on common stock	0	0	0
Tax expense from employee stock option plan	0	0	0
Stock-based compensation	0	0	0
Issuance of common stock	0	0	0
Common stock expense		0
Retained Earnings [Member]
Balance	385,753	373,905	356,727	328,712
Comprehensive income	59,855	63,898	72,667
Dividends paid on common stock	(48,007)	(46,690)	(44,652)
Tax expense from employee stock option plan	0	0	0
Stock-based compensation	0	0	0
Issuance of common stock	0	0	0
Common stock expense		$ (30)