Earnings per share (Details) - shares shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Weighted Average Number of Shares Outstanding Reconciliation
Weighted average number of common shares outstanding used to compute basic earnings per share	51,978	54,372	51,749	54,239
Dilutive effect of stock-based awards	655	801	686	859
Number of shares used to compute diluted earnings per share	52,633	55,173	52,435	55,098