Revenue Recognition (Schedule of Consolidated Balance Sheet) (Details) (Parenthetical) - $ / shares	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Preferred Stock, par value per share	$ 5.00	$ 5.00
Preferred Stock, shares authorized	2,000,000	2,000,000
Preferred Stock, shares issued	0	0
Common Class A [Member]
Common Stock, par value per share	$ 0.001	$ 0.001
Common Stock, shares authorized	100,000,000	100,000,000
Common Stock, shares issued	37,450,329	37,395,753
Treasury stock, shares	8,418,620	8,431,335
Common Class B [Member]
Common Stock, par value per share	$ 0.001	$ 0.001
Common Stock, shares authorized	25,000,000	25,000,000
Common Stock, shares issued	3,233,998	3,233,998
Common Stock, shares outstanding	3,233,998	3,233,998
As previously reported [Member]
Preferred Stock, par value per share		$ 5.00
Preferred Stock, shares authorized		2,000,000
Preferred Stock, shares issued		0
As previously reported [Member] | Common Class A [Member]
Common Stock, par value per share		$ 0.001
Common Stock, shares authorized		100,000,000
Common Stock, shares issued		37,395,753	37,319,266
Treasury stock, shares		8,431,335	8,443,444
As previously reported [Member] | Common Class B [Member]
Common Stock, par value per share		$ 0.001
Common Stock, shares authorized		25,000,000
Common Stock, shares issued		3,233,998	3,233,998
Common Stock, shares outstanding		3,233,998	3,233,998
Adjustments Increase/(decrease) [Member]
Preferred Stock, par value per share		$ 5.00
Preferred Stock, shares authorized		2,000,000
Preferred Stock, shares issued		0
Adjustments Increase/(decrease) [Member] | Common Class A [Member]
Common Stock, par value per share		$ 0.001
Common Stock, shares authorized		100,000,000
Common Stock, shares issued		37,395,753	37,319,266
Adjustments Increase/(decrease) [Member] | Common Class B [Member]
Common Stock, par value per share		$ 0.001
Common Stock, shares authorized		25,000,000
Common Stock, shares issued		3,233,998	3,233,998
Common Stock, shares outstanding		3,233,998	3,233,998
Opening balance, as adjusted, January 1, 2018 [Member]
Preferred Stock, par value per share		$ 5.00
Preferred Stock, shares authorized		2,000,000
Preferred Stock, shares issued		0
Opening balance, as adjusted, January 1, 2018 [Member] | Common Class A [Member]
Common Stock, par value per share		$ 0.001
Common Stock, shares authorized		100,000,000
Common Stock, shares issued		37,395,753	37,319,266
Opening balance, as adjusted, January 1, 2018 [Member] | Common Class B [Member]
Common Stock, par value per share		$ 0.001
Common Stock, shares authorized		25,000,000
Common Stock, shares issued		3,233,998	3,233,998
Common Stock, shares outstanding		3,233,998	3,233,998