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Summary of significant accounting policies (Earnings per share reconciliation) (Details) - shares
shares in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]                      
Basic weighted average shares outstanding (in shares) 91,923 90,860 90,707 90,477 90,175 90,058 89,767 89,482 91,226 89,908 92,628
Dilutive effect of stock option and restricted share plan (in shares)                 0 0 2,680
Diluted weighted average shares outstanding (in shares) 91,923 90,860 90,707 90,477 90,175 91,687 91,884 91,469 91,226 89,908 95,308