Employee benefits (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Term of Stock Options	10
Proceeds from stock options exercised	$ 0	$ 22,000	$ 645,000
Restricted stock, shares issued	326,000	525,000	412,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options outstanding at beginning of year (in shares)	448,000	897,000	1,172,000
Forfeited (in shares)	(101,000)	(171,000)	(34,000)
Options outstanding at end of year (in shares)	347,000	448,000	897,000
Year-end weighted average remaining life of options (years)	2 years 2 months	3 years	4 years 3 months
Options available for future grants (in shares)	767,000	575,000	406,000
Weighted average exercise price of options: [Abstract]
Exercised during year (in dollars per share)	$ 0	$ 2.17	$ 2.79
Forfeited during year (in dollars per share)	$ 6.34	$ 9.16	$ 8.07
Outstanding at end of year (in dollars per share)	$ 8.46	$ 7.98	$ 6.40
Exercisable at end of year (in dollars per share)	$ 8.46	$ 8.01	$ 6.41
Intrinsic value - options outstanding at end of year	0	0	423,000
Intrinsic value - options exercisable at end of year	0	0	420,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding shares at beginning of year (in shares)	368,196	375,681	374,348
Issued (in shares)	253,400	137,001	127,640
Forfeited (in shares)	(1,000)	(14,800)	(17,834)
Vested (in shares)	(138,749)	(129,686)	(108,473)
Outstanding shares at end of year (in shares)	481,847	368,196	375,681
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Intrinsic Value [Roll Forward]
Outstanding at beginning of year	1,328,000	1,330,000	1,468,000
Issued	326,000	525,000	412,000
Forfeited	(4,000)	(53,000)	(73,000)
Vested	(497,000)	(474,000)	(477,000)
Outstanding at end of year	1,153,000	1,328,000	1,330,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Outstanding at beginning of year (in dollars per share)	$ 3.61	$ 3.54	$ 3.93
Issued (in dollars per share)	$ 1.29	$ 3.83	$ 3.23
Forfeited (in dollars per share)	$ 4.00	$ 3.56	$ 4.10
Vested (in dollars per share)	$ 3.58	$ 3.65	$ 4.40
Outstanding at end of year (in dollars per share)	$ 2.39	$ 3.61	$ 3.54
Share-based compensation expense	568,000	470,000	444,000
Unrecognized share-based compensation expense	800,000
Weighted average period unrecognized compensation expense is to be recognized	19 months
Supplemental Employee Retirement Plan [Abstract]
Supplemental employee retirement plan liability	665,000
Shares in supplemental employee retirement plan trust (in shares)	111,000
Shares purchased for supplemental employee retirement plan (in shares)	100,000
Value of shares purchased by SERP participants	155,000
Shares liquidated for SERP (in shares)	59,000
Value of Shares Liquidated	87,000
Defined Benefit Plan Disclosure [Line Items]
Percentage of employer matching contribution (in hundredths)	25.00%
Percentage of employee's annual salary (in hundredths)	4.00%
Defined benefit plan, vesting period	6
Rights Agreement [Abstract]
Number of shares of common stock each right entitles holder (in shares)	1
Exercise price of rights (in dollars per share)	$ 11.00
Number of times exercise price of rights	2
Price rights are redeemed (in dollars per share)	$ 0.001
Period Rights Agreement is Reviewed	3
Participants Under the Age of 50 [Member]
Defined Benefit Plan Disclosure [Line Items]
Maximum employee contribution per year	17,000
Participants Over the Age of 50 [Member]
Defined Benefit Plan Disclosure [Line Items]
Maximum employee contribution per year	22,500
$1.50 to $5.00 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Number of options outstanding (in thousands)	58,000
Weighted average remaining contractual life (years)	4 months
Weighted average exercise price (in dollars per share)	$ 2.32
$5.01 to $8.00 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Number of options outstanding (in thousands)	67,000
Weighted average remaining contractual life (years)	1 year 3 months
Weighted average exercise price (in dollars per share)	$ 6.64
$8.01 to $12.00 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Number of options outstanding (in thousands)	222,000
Weighted average remaining contractual life (years)	2 years 9 months
Weighted average exercise price (in dollars per share)	$ 10.60
Total Price Range [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Number of options outstanding (in thousands)	347,000
Weighted average remaining contractual life (years)	2 years 2 months
Weighted average exercise price (in dollars per share)	$ 8.46
Director [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock, shares issued (in shares)	124,000	53,000	28,000
Restricted stock, shares issued	174,000	196,000	102,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Intrinsic Value [Roll Forward]
Issued	174,000	196,000	102,000
Employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock, shares issued (in shares)	129,000	84,000	100,000
Restricted stock, shares issued	159,000	320,000	310,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Intrinsic Value [Roll Forward]
Issued	$ 159,000	$ 320,000	$ 310,000
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	5 years
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Exercised (in shares)	0	(278,000)	(241,000)