Equity Plans Equity Plans (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 purchase_interval	Dec. 31, 2011	Dec. 31, 2010
Summary of Activity under the Plan [Abstract]
Compensation costs charged to income	$ 564	$ 651	$ 812
Proceeds from the exercise of stock options and similar instruments	989	564	692
Tax benefits realized from share options exercised	97	7	41
Employee Stock Purchase Plan [Abstract]
Number of purchases intervals under the plan since inception	13
Shares issued during the period under the plan	29,468	25,712	25,860
Cash received from employees toward purchases under the plan	627	548	512
Restricted Stock and Deferred Restricted Stock [Member]
Summary of Activity under the Plan [Abstract]
Compensation costs charged to income	460	559	722
Tax benefits realized from share options exercised	36	7	0
Employee Stock Purchase Plan [Abstract]
Unrecognized compensation cost	622
Deferred Restricted Stock Awards [Member]
Dividend Equivalent Rights [Abstract]
Cumulative dividend equivalent rights converted to deferred restricted stock awards	56,349	50,888	45,731
Employee Stock Option [Member]
Summary of Activity under the Plan [Abstract]
Proceeds from the exercise of stock options and similar instruments	318	0	180
Tax benefits realized from share options exercised	61	0	41
Dividend Equivalent Rights [Member]
Dividend Equivalent Rights [Abstract]
Liability for dividend equivalent rights	128	129	123
Incentive Plan [Member]
Option Indexed to Issuer's Equity [Line Items]
Number of shares reserved for issuance under the plan	1,800,000
Maximum number of shares per employee	600,000
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued Pursuant To The Plan	287,534	235,473	213,207
Shares issuable under the plan upon exercise of outstanding awards	327,093	363,700	378,903
Number of shares available for grant	1,185,373	1,200,827	1,207,890
Dividend Reinvestment and Stock Purchase Plan [Member]
Option Indexed to Issuer's Equity [Line Items]
Number of shares reserved for issuance under the plan	3,000,000
Shares issued under the plan	3,487	1,679
Summary of Activity under the Plan [Abstract]
Proceeds from the exercise of stock options and similar instruments	83	39	0
Employee Stock [Member]
Option Indexed to Issuer's Equity [Line Items]
Number of shares reserved for issuance under the plan	270,400
Summary of Activity under the Plan [Abstract]
Compensation costs charged to income	104	92	90
Proceeds from the exercise of stock options and similar instruments	588	525	512
Employee Stock Purchase Plan [Abstract]
Purchase price of common stock to employees under the plan, percent	85.00%
Maximum amount of base salary employees can designate for share purchase under the plan	10.00%
Expenses recorded under the plan	110	96	90
Unrecognized compensation cost	$ 52