Common Stock and Treasury Stock (Details) - shares shares in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Class A Common Stock [Member]
Common Stock [Line Items]
Balance, shares	6,694	6,638
Conversion of shares		17
Issuance of restricted stock	4	19
Forfeiture of restricted stock		(1)
Exercise of stock options		21
Balance, shares	6,698	6,694
Class B Common Stock [Member]
Common Stock [Line Items]
Balance, shares	898	878
Conversion of shares		(17)
Issuance of restricted stock	0	29
Forfeiture of restricted stock		0
Exercise of stock options		8
Balance, shares	898	898