Shareholders' equity (Details 2) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended			12 Months Ended	15 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2015
Aggregate intrinsic value
Weighted average remaining life (years)		8 years 9 months 18 days		9 years 3 months 18 days
Weighted average exercisable (years)	7 years 9 months 18 days		8 years 6 months
Stock options
Class of Stock
Vesting period	4 years		4 years
Expiration period	10 years		10 years
Restricted stock units
Class of Stock
Vesting period					4 years
Expiration period					10 years
Stock Options
Outstanding at beginning of the period (in shares)	85,000
Granted (in shares)	42,000		92,000
Exercised (in shares)	(21,250)
Forfeited (in shares)	(6,000)		(2,000)
Outstanding at end of the period (in shares)	99,750		90,000		99,750
2012 Equity Incentive Plan | Stock options
Stock Options
Outstanding at beginning of the period (in shares)	2,165,000		1,190,000		1,190,000
Granted (in shares)	553,500		970,000
Forfeited (in shares)	(110,500)		(41,250)
Outstanding at end of the period (in shares)	2,608,000		2,118,750		2,608,000
Exercisable at end of the period (in shares)	597,500		160,000		597,500
Weighted Average Exercise Price
Outstanding at beginning of the period (in dollars per share)	16.17		15.48		15.48
Granted (in dollars per share)	16.48		16.63
Forfeited (in dollars per share)	16.57		16.18
Outstanding at end of the period (in dollars per share)	16.22		15.99		16.22
Exercisable at end of the period (in dollars per share)	15.80		15.00		15.80
Aggregate intrinsic value
Weighted average remaining life (years)	8 years 8 months 12 days		9 years 4 months 24 days
Aggregate intrinsic value	1,064	1,890	1,532	862	1,064
Aggregate intrinsic value exercisable	466		274		466
2012 Equity Incentive Plan | Restricted stock units
Class of Stock
Stock options granted	42,000
2012 Equity Incentive Plan | Class A common shares
Class of Stock
Stock options granted	553,500		970,000