Incentive plans (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Stock option [Roll Forward]
Exercised (in shares)	0	0
Information relating to 1993 Plan [Abstract]
Common shares reserved for issuance (in shares)	4,430
1993 Stock Incentive Plan [Member]
Stock option [Roll Forward]
Outstanding at beginning of period (in shares)	0	0
Exercised (in shares)	0	0
Granted (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]
Outstanding at beginning of period (in dollars per share)	$ 0	$ 0
Exercised (in dollars per share)	0	0
Granted (in dollars per share)	0	0
Expired (in dollars per share)	0	0
Outstanding at beginning of period (in dollars per share)	0	0
Exercisable at end of period (in dollars per share)	$ 0	$ 0
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 (in shares)	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