Consolidated Statements of Changes in Shareholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]
Balance, value at Dec. 31, 2009	$ 252,756	$ 9,635	$ 22,046	$ 207,888	$ 13,187
Balance, shares at Dec. 31, 2009		18,499,602
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	24,406	0	0	24,406	0
Unrealized loss on investment, net of tax effect	(44)	0	0	0	(44)
Reclassification adjustment for gain realized on investment, net of tax effect of $7,494	(10,784)	0	0	0	(10,784)
Share-based compensation	689	0	812	(123)	0
Exercise of stock options and similar instruments, value	100	14	86	0	0
Exercise of stock options and similar instruments, shares		26,078
Employee stock purchase plan, value	512	13	499	0	0
Dividends	(12,603)	0	0	(12,603)	0
Employee stock purchase plan, shares	25,860	25,860
Balance, value at Dec. 31, 2010	255,032	9,662	23,443	219,568	2,359
Balance, shares at Dec. 31, 2010		18,551,540
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	20,878	0	0	20,878	0
Unrealized loss on investment, net of tax effect	(85)	0	0	0	(85)
Reclassification adjustment for gain realized on investment, net of tax effect of $7,494	0
Share-based compensation	522	0	651	(129)	0
Exercise of stock options and similar instruments, value	(84)	7	(91)	0	0
Exercise of stock options and similar instruments, shares		13,896
Employee stock purchase plan, value	525	14	511	0	0
Dividends	(12,823)	0	0	(12,823)	0
Employee stock purchase plan, shares	25,712	25,712
Dividend reinvestment and stock purchase plan, value	39	1	38	0	0
Dividend reinvestment and stock purchase plan, shares		1,679
Balance, value at Dec. 31, 2011	264,004	9,684	24,552	227,494	2,274
Balance, shares at Dec. 31, 2011	18,592,827	18,592,827
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	22,318	0	0		0
Unrealized loss on investment, net of tax effect	36	0	0	0	36
Reclassification adjustment for gain realized on investment, net of tax effect of $7,494	0
Share-based compensation	436	0	564	(128)	0
Exercise of stock options and similar instruments, value	370	23	347	0	0
Exercise of stock options and similar instruments, shares		29,468
Employee stock purchase plan, value	588	15	573	0	0
Dividends	(13,231)	0	0	(13,231)	0
Employee stock purchase plan, shares	29,468	44,784
Dividend reinvestment and stock purchase plan, value	83	2	81	0	0
Dividend reinvestment and stock purchase plan, shares		3,487
Balance, value at Dec. 31, 2012	$ 274,604	$ 9,724	$ 26,117	$ 236,453	$ 2,310
Balance, shares at Dec. 31, 2012	18,670,566	18,670,566