Common shares - Additional Information (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Equity [Abstract]
Options outstanding	557,750	742,800	841,900
Term of option	10 years
Vesting period	5 years
Weighted average estimated fair value at the date of the grant for the options granted during respective period	$ 3.82	$ 3.54	$ 2.96
Compensation expense related to stock options	$ 270	$ 431	$ 501