Common Shares (Details) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions			3 Months Ended		12 Months Ended
	Apr. 03, 2018	Feb. 23, 2018	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Common Shares.
Shares repurchased					3.1
Total aggregate amount of shares purchased					$ 18,100
Common shares
Common stock dividend declared (in dollars per share)		$ 0.06	$ 0.06	$ 0.06
Cash dividends paid	$ 19,100		$ 17,148	$ 17,040