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, 2008	$ 254,326	$ 9,611	$ 20,548	$ 204,744	$ 19,423
Balance, shares at Dec. 31, 2008		18,452,447
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	15,171	0	0	15,171	0
Unrealized loss on investment, net of tax effect	(6,236)	0	0	0	(6,236)
Reclassification adjustment for gain realized on investment, net of tax effect	0
Share-based compensation	775	0	897	(122)	0
Exercise of stock options and similar instruments, value	113	11	102	0	0
Exercise of stock options and similar instruments, shares		22,481
Employee stock purchase plan, value	512	13	499	0	0
Employee stock purchase plan, shares		24,674
Adjustment to equity interest in CLWSC	297	0	0	297	0
Dividends paid	(12,202)	0	0	(12,202)	0
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	(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
Employee stock purchase plan, shares		25,860
Dividends paid	(12,603)	0	0	(12,603)	0
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	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
Reclassification adjustment for gain realized on investment, net of tax effect	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
Employee stock purchase plan, shares		25,712
Dividend reinvestment and stock purchase plan, value	39	1	38	0	0
Dividend reinvestment and stock purchase plan, shares		1,679
Dividends paid	(12,823)	0	0	(12,823)	0
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