Stock plans (Components of share compensation expenses) (Details) - USD ($) $ 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
Total share compensation expenses
Share compensation expenses	$ 10,442	$ 10,501	$ 10,727	$ 11,237	$ 10,062	$ 9,983	$ 9,242	$ 9,054	$ 42,907	$ 38,341	$ 33,073
Restricted share awards [Member]
Total share compensation expenses
Share compensation expenses									36,887	35,386	31,335
Restricted share units [Member]
Total share compensation expenses
Share compensation expenses									1,285	1,160	876
Performance Shares [Member]
Total share compensation expenses
Share compensation expenses									$ 4,735	$ 1,795	$ 862