Stock incentive plans, Stock options (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of shares [Roll Forward]
Options vested or expected to vest (in shares)	95,135
Weighted average remaining contractual term [Abstract]
Options vested or expected to vest	2 years
Aggregate intrinsic value [Abstract]
Unrecognized compensation cost related to stock options	$ 899,000
Weighted average period to recognize compensation cost not yet recognized	2 years 7 months 6 days
Intrinsic value of stock options exercised	$ 958,000	$ 6,000	$ 32,000
Fair value of all awards vested	1,104,000	961,000	944,000
Cash received from exercise of stock options	1,043,000	23,000	263,000
Employee Service Share-based Compensation, Tax Benefit from Exercise of Stock Options	$ 150,000	$ 1,000	$ 4,000
Stock Options [Member]
Number of shares [Roll Forward]
Options exercisable, end of period (in shares)	483,224
Options vested or expected to vest (in shares)	919,639
2005 Equity Incentive Plan and 2014 Equity Incentive Plan [Member]
Number of shares [Roll Forward]
Outstanding, beginning of period (in shares)	57,400
Granted (in shares)	65,600
Exercised (in shares)	(8,300)
Forfeited (in shares)	(13,425)
Expired (in shares)	0
Outstanding, end of period (in shares)	101,275	57,400
Weighted average exercise price [Abstract]
Outstanding, beginning of period (in dollars per share)	$ 7.94
Granted (in dollars per share)	8.30
Exercised (in dollars per share)	7.44
Forfeited (in dollars per share)	7.98
Expired (in dollars per share)	0
Outstanding, end of period (in dollars per share)	$ 8.21	$ 7.94
Options vested or expected to vest (in dollars per share)		$ 0
Aggregate intrinsic value [Abstract]
Options vested or expected to vest	$ 1,261,000
2005 Equity Incentive Plan and 2014 Equity Incentive Plan [Member] | Stock Options [Member]
Number of shares [Roll Forward]
Outstanding, beginning of period (in shares)	1,073,900
Granted (in shares)	169,500
Exercised (in shares)	(182,811)
Forfeited (in shares)	(80,763)
Expired (in shares)	(60,187)
Outstanding, end of period (in shares)	919,639	1,073,900
Weighted average exercise price [Abstract]
Outstanding, beginning of period (in dollars per share)	$ 7.83
Granted (in dollars per share)	8.30
Exercised (in dollars per share)	6.42
Forfeited (in dollars per share)	7.50
Expired (in dollars per share)	9.32
Outstanding, end of period (in dollars per share)	8.13	$ 7.83
Options exercisable, end of period (in dollars per share)	$ 8.29
Options vested or expected to vest (in dollars per share)		$ 8.13
Weighted average remaining contractual term [Abstract]
Options exercisable, end of period		4 years 9 months 18 days
Options vested or expected to vest		6 years 4 months 24 days
Aggregate intrinsic value [Abstract]
Options exercisable, end of period	$ 2,396,000
Options vested or expected to vest	$ 4,709,000