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			9 Months Ended	12 Months Ended
	Sep. 30, 2019	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Dec. 31, 2018
Class A Common Stock
Common Stock [Line Items]
Balance, shares	6,732	6,698	6,694	6,732	6,694
Conversion of shares					12
Issuance of restricted stock			4		27
Forfeiture of restricted stock (in shares)	(2)	(1)		(2)	(1)
Balance, shares	6,730	6,697	6,698	6,730	6,732
Class B Common Stock
Common Stock [Line Items]
Balance, shares	923	898	898	923	898
Conversion of shares					(12)
Issuance of restricted stock					37
Forfeiture of restricted stock (in shares)					0
Balance, shares	923	898	898	923	923