Earnings per share attributable to stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Net income attributable to:
Basic	$ 53,043	$ 54,036	$ 75,851	$ 72,200
Diluted	$ 53,043	$ 54,036	$ 75,851	$ 72,200
Weighted average number of shares outstanding:
Basic	108,387,490	108,707,708	108,818,903	108,725,871
Diluted	109,323,343	109,942,768	109,903,808	109,982,763
Per Share Amount
Basic	$ 0.49	$ 0.50	$ 0.70	$ 0.66
Diluted	$ 0.49	$ 0.49	$ 0.69	$ 0.66
Share Units [Member]
Weighted average number of shares outstanding:
Effect of dilutive securities:	424,812	442,601	505,443	464,613
Stock Options [Member]
Weighted average number of shares outstanding:
Effect of dilutive securities:	511,041	792,459	579,462	792,279
Per Share Amount
Effect of dilutive securities:		(0.01)	(0.01)