Earnings per share attributable to stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net income attributable to:
Basic	$ 45,387	$ 25,266	$ 121,239	$ 97,466
Diluted	$ 45,387	$ 25,266	$ 121,239	$ 97,466
Weighted average number of shares outstanding:
Basic	109,018,469	108,003,390	108,887,026	108,453,525
Diluted	110,369,718	109,381,173	110,060,712	109,634,195
Per Share Amount
Basic	$ 0.42	$ 0.23	$ 1.11	$ 0.90
Diluted	$ 0.41	$ 0.23	$ 1.10	$ 0.89
Share Units [Member]
Weighted average number of shares outstanding:
Effect of dilutive securities:	548,859	481,268	519,915	430,175
Stock Options [Member]
Weighted average number of shares outstanding:
Effect of dilutive securities:	802,390	896,515	653,771	750,495
Per Share Amount
Effect of dilutive securities:	(0.01)		(0.01)	(0.01)