Common Stock and Treasury Stock - Information relating to the number of shares of our common stock issued (Details) - shares shares in Thousands	3 Months Ended		12 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2018
Class A Common Stock
Common Stock [Line Items]
Balance, shares	6,698	6,694	6,694
Conversion of shares			12
Issuance of restricted stock		4	27
Forfeiture of restricted stock (in shares)	(1)		(1)
Balance, shares	6,697	6,698	6,732
Class B Common Stock
Common Stock [Line Items]
Balance, shares	898	898	898
Conversion of shares			(12)
Issuance of restricted stock		0	37
Forfeiture of restricted stock (in shares)			0
Balance, shares	898	898	923