Earnings per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of numerator and denominator of the basic and diluted earnings per share computations [Abstract]
Net income									$ 63,316	$ 33,787	$ 24,901
Basic earnings per share [Abstract]
Earnings per share - basic	$ 0.81	$ 0.60	$ 0.65	$ 0.88	$ 0.21	$ 0.57	$ 0.48	$ 0.35	$ 2.94	$ 1.60	$ 1.19
Diluted earnings per share [Abstract]
Earnings per share - diluted	$ 0.79	$ 0.58	$ 0.63	$ 0.86	$ 0.20	$ 0.55	$ 0.47	$ 0.34	$ 2.86	$ 1.56	$ 1.16
Basic weighted average shares outstanding [Abstract]
Weighted average shares outstanding - basic									21,528,000	21,090,000	20,992,000
Dilutive effect of shares issued under stock compensation arrangements [Abstract]
Dilutive effect of shares issued under stock compensation arrangements (in shares)									608,000	595,000	506,000
Diluted weighted average shares outstanding [Abstract]
Weighted average shares outstanding - diluted (in shares)									22,136,000	21,685,000	21,498,000
Antidilutive securities (in shares)									54,000		0
Stock options [Member]
Dilutive effect of shares issued under stock compensation arrangements [Abstract]
Dilutive effect of shares issued under stock compensation arrangements (in shares)									570,000	591,000	506,000
Restricted Stock [Member]
Dilutive effect of shares issued under stock compensation arrangements [Abstract]
Dilutive effect of shares issued under stock compensation arrangements (in shares)									38,000	4,000	0