Incentive plans (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Stock option [Roll Forward]
Exercised (in shares)	0	0
Information relating to 1993 Plan [Abstract]
Options to purchase shares of common stock which were excluded from computation of diluted earnings per share due to the effect of being anti-dilutive in the computation of earnings per share.	0	0
Common shares reserved for issuance (in shares)	4,430
1993 Stock Incentive Plan [Member]
Information relating to 1993 Plan [Abstract]
Unamortized compensation cost relating to non-vested stock options	0	0
Stock based compensation expense recorded for the year ended	0	0
Options to purchase shares of common stock which were excluded from computation of diluted earnings per share due to the effect of being anti-dilutive in the computation of earnings per share.	0
Common shares reserved for issuance (in shares)	4,320
Shares available for future stock option grants (in shares)	4,320
Intrinsic value of options outstanding	0
Intrinsic value of options exercisable	0
1993 Stock Incentive Plan [Member] | Stock Options [Member]
Stock option [Roll Forward]
Outstanding at beginning of period (in shares)	0	0
Exercised (in shares)	0	0
Expired (in shares)	0	0
Outstanding at end of period (in shares)	0	0
Exercisable at end of period (in shares)	0	0
Weighted Average Exercise Price [Roll Forward]
Weighted average exercise price outstanding at beginning of period (in dollars per share)	0	0
Weighted average exercise price, exercised (in dollars per share)	0	0
Weighted average exercise price, expired (in dollars per share)	0	0
Weighted average exercise price outstanding at beginning of period (in dollars per share)	0	0
Weighted average exercise price, exercisable at end of period (in dollars per share)	0	0