Earnings per share attributable to stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	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, 2020	Dec. 31, 2019	Dec. 31, 2018
Net income attributable to:
Basic													$ 170,095	$ 149,039	$ 121,479
Diluted													$ 170,095	$ 149,039	$ 121,479
Weighted average number of shares outstanding:
Basic													109,054,493	108,519,739	108,063,349
Diluted													110,310,984	109,759,123	109,388,236
Per Share Amount
Basic	$ 0.45	$ 0.42	$ 0.49	$ 0.21	$ 0.47	$ 0.23	$ 0.50	$ 0.17	$ 0.33	$ 0.21	$ 0.42	$ 0.16	$ 1.56	$ 1.37	$ 1.12
Diluted	$ 0.44	$ 0.41	$ 0.49	$ 0.21	$ 0.47	$ 0.23	$ 0.49	$ 0.17	$ 0.32	$ 0.21	$ 0.42	$ 0.16	$ 1.54	$ 1.36	$ 1.11
Share Units [Member]
Weighted average number of shares outstanding:
Effect of dilutive securities:													541,054	458,763	459,503
Per Share Amount
Effect of dilutive securities:													(0.01)
Stock Options [Member]
Weighted average number of shares outstanding:
Effect of dilutive securities:													715,437	780,621	865,384
Per Share Amount
Effect of dilutive securities:													(0.01)	(0.01)	(0.01)