Earnings per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Computation of basic and diluted earnings per share
Net income	$ 6,862	$ 6,950	$ 15,802	$ 16,341
Weighted average common shares outstanding - basic	87,299,000	87,736,000	88,019,000	87,660,000
Plus: net effect of dilutive stock options	94,000	272,000	157,000	293,000
Plus: net effect of dilutive restricted stock awards and units	399,000	225,000	275,000	192,000
Weighted average common shares outstanding, diluted (in shares)	87,792,000	88,233,000	88,451,000	88,145,000
Net income per common share-basic (in dollars per share)	$ 0.08	$ 0.08	$ 0.18	$ 0.19
Net income per common share-diluted (in dollars per share)	$ 0.08	$ 0.08	$ 0.18	$ 0.19
Stock Options
Computation of basic and diluted earnings per share
Anti-dilutive securities excluded from diluted earnings per share calculations (in shares)	3,708,228	2,251,303	3,490,887	2,395,158
Restricted Stock Units
Computation of basic and diluted earnings per share
Anti-dilutive securities excluded from diluted earnings per share calculations (in shares)	71,581	2,367	169,682	79,947