Consolidated Balance Sheets Parentheticals - $ / shares	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Class A Common Shares [Member]
Common Stock, par value per share	$ 0.01	$ 0.01
Common Stock, votes per share	1	1
Common Stock, shares authorized	180,000,000	180,000,000
Common Stock, shares issued	74,996,304	74,808,305
Common Stock, shares outstanding	59,428,675	61,946,078
Treasury Stock, shares	15,567,629	12,862,227
Class B Common Shares [Member]
Common Stock, par value per share	$ 0.01	$ 0.01
Common Stock, votes per share	10	10
Common Stock, shares authorized	30,000,000	30,000,000
Common Stock, shares issued	671,138	701,138
Common Stock, shares outstanding	671,138	701,138