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, 2011	$ 264,004us-gaap_StockholdersEquity	$ 9,684	$ 24,552	$ 227,494	$ 2,274
Balance, shares at Dec. 31, 2011		18,592,827
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	22,318us-gaap_NetIncomeLoss	0	0	22,318	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 $805	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		44,784
Employee stock purchase plan, value	588	15	573	0	0
Employee stock purchase plan, shares	29,468	29,468
Dividend reinvestment and stock purchase plan, value	83	2	81	0	0
Dividend reinvestment and stock purchase plan, shares		3,487
Dividends	(13,231)us-gaap_Dividends	0	0	(13,231)	0
Balance, value at Dec. 31, 2012	274,604us-gaap_StockholdersEquity	9,724	26,117	236,453	2,310
Balance, shares at Dec. 31, 2012		18,670,566
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	22,384us-gaap_NetIncomeLoss	0	0	22,384	0
Unrealized loss on investment, net of tax effect	1,077	0	0	0	1,077
Reclassification adjustment for gain realized on investment, net of tax effect of $805	0
Share-based compensation	784	0	912	(128)	0
Exercise of stock options and similar instruments, value	69	23	46	0	0
Exercise of stock options and similar instruments, shares		43,665
Employee stock purchase plan, value	722	16	706	0	0
Employee stock purchase plan, shares	30,869	30,869
Dividend reinvestment and stock purchase plan, value	84	2	82	0	0
Dividend reinvestment and stock purchase plan, shares		3,111
Common stock issued, shares		1,421,000
Common stock issued	35,894	740	35,154	0	0
Dividends	(14,443)us-gaap_Dividends	0	0	(14,443)	0
Balance, value at Dec. 31, 2013	321,175us-gaap_StockholdersEquity	10,505	63,017	244,266	3,387
Balance, shares at Dec. 31, 2013	20,169,211us-gaap_CommonStockSharesOutstanding	20,169,211
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	51,806us-gaap_NetIncomeLoss	0	0	51,806	0
Unrealized loss on investment, net of tax effect	301	0	0	0	301
Reclassification adjustment for gain realized on investment, net of tax effect of $805	(1,171)	0	0	0	(1,171)
Share-based compensation	909	0	1,031	(122)	0
Exercise of stock options and similar instruments, value	1,439	42	1,397	0	0
Exercise of stock options and similar instruments, shares		80,796
Employee stock purchase plan, value	839	19	820	0	0
Employee stock purchase plan, shares	35,682	35,682
Dividend reinvestment and stock purchase plan, value	34	1	33	0	0
Dividend reinvestment and stock purchase plan, shares		1,151
Dividends	(15,177)us-gaap_Dividends	0	0	(15,177)	0
Balance, value at Dec. 31, 2014	$ 360,155us-gaap_StockholdersEquity	$ 10,567	$ 66,298	$ 280,773	$ 2,517
Balance, shares at Dec. 31, 2014	20,286,840us-gaap_CommonStockSharesOutstanding	20,286,840

This element represents the amount of recognized equity-based compensation during the period, that is, the amount recognized as expense in the income statement (or as asset if compensation is capitalized). Alternate captions include the words "stock-based compensation".

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 35

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6415241&loc=d3e4534-113899

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 20

-Section 55

-Paragraph 12

-URI http://asc.fasb.org/extlink&oid=32706628&loc=d3e11149-113907

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 20

-Section 55

-Paragraph 13

-URI http://asc.fasb.org/extlink&oid=32706628&loc=d3e11178-113907