Earnings per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Computation of basic and diluted earnings per share
Net income	$ 5,418	$ 6,038	$ 8,940	$ 9,391
Weighted average common shares outstanding - basic	88,523,000	87,691,000	88,385,000	87,564,000
Plus: net effect of dilutive stock options	153,000	230,000	167,000	301,000
Plus: net effect of dilutive restricted stock awards and units	232,000	171,000	219,000	168,000
Weighted average common shares outstanding, diluted (in shares)	88,908,000	88,092,000	88,771,000	88,033,000
Net income per common share-basic (in dollars per share)	$ 0.06	$ 0.07	$ 0.10	$ 0.11
Net income per common share-diluted (in dollars per share)	$ 0.06	$ 0.07	$ 0.10	$ 0.11
Stock Options
Computation of basic and diluted earnings per share
Anti-dilutive securities excluded from diluted earnings per share calculations (in shares)	2,576,385	2,562,229	2,969,496	2,443,375
Restricted Stock Units
Computation of basic and diluted earnings per share
Anti-dilutive securities excluded from diluted earnings per share calculations (in shares)	48,131	9,085	127,185	52,171