CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other comprehensive income (loss), net of tax:
Dividends declared and paid: (in dollars per share)	$ 1.20	$ 1.00	$ 0.92
Repurchase of Class A Common shares (in shares)	0	0	3,986,275
Convert Voting Shares to Class A Common Shares (in shares)			466,690
Exercise of employee share options: shares issued (in shares)	483,195	351,973	285,938
Other share-based compensation, net: shares issued (in shares)	331,513	450,005	515,010
Other share-based compensation, net: shares repurchased (in shares)	101,558	149,101	164,104