Earnings (loss) per common share (Details 7) - shares shares in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Weighted average common shares outstanding - basic	200,383	199,343	198,270	196,401	196,023	196,018	195,524	195,304	198,612	195,720	195,304
Effect of dilutive performance share awards									14	430	383
Weighted average common shares outstanding - diluted	200,478	199,383	198,287	196,414	196,385	196,265	196,169	195,982	198,626	196,150	195,687
Shares excluded from the calculation of diluted earnings per share									164	10	0