Consolidated Balance Sheets (Parenthetical) (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Trade receivables, related parties	$ 5,145	$ 4,530
Trade receivables, allowance for doubtful accounts	3,032	3,395
Prepaid expenses and other current assets	12,558	10,672
Property and equipment, accumulated depreciation	187,226	187,375
Accumulated amortization of Intangible assets	95,646	93,793
Intangible assets subject to amortization, net	19,442	22,598
Accounts payable and accrued expenses	46,198	51,034
Unamortized debt issuance costs	2,718	2,709
Related Parties
Prepaid expenses and other current assets	274	274
Intangible assets subject to amortization, net	7,712	8,327
Accounts payable and accrued expenses	$ 2,415	$ 1,948
Class A common stock
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	260,000,000	260,000,000
Common stock, shares issued	60,718,613	63,210,531
Common stock, shares outstanding	60,718,613	63,210,531
Class B common stock
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	40,000,000	40,000,000
Common stock, shares issued	14,927,613	14,927,613
Common stock, shares outstanding	14,927,613	14,927,613
Class U common stock
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	40,000,000	40,000,000
Common stock, shares issued	9,352,729	9,352,729
Common stock, shares outstanding	9,352,729	9,352,729