Common Stock and Treasury Stock (Details) - shares shares in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Common Class A [Member]
Common Stock [Line Items]
Balance, shares	6,732	6,694	6,694
Conversion of shares			12
Issuance of restricted stock	0	4	27
Forfeiture of restricted stock	0		(1)
Balance, shares	6,732	6,698	6,732
Common Class B [Member]
Common Stock [Line Items]
Balance, shares	923	898	898
Conversion of shares			(12)
Issuance of restricted stock	0	0	37
Forfeiture of restricted stock	0		0
Balance, shares	923	898	923