Earnings per share (Details) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Weighted Average Number of Shares Outstanding Reconciliation
Weighted average number of common shares outstanding used to compute basic earnings per share	52,654	53,374	52,053	53,955
Dilutive effect of stock-based awards	650	755	706	849
Number of shares used to compute diluted earnings per share	53,304	54,129	52,759	54,804