Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Allowance for doubtful accounts receivable	$ 7,390	$ 6,739
Accumulated amortization of finite-lived intangible assets	$ 1,011,145	$ 953,957
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	400,000,000	400,000,000
Common stock, shares issued	209,375,921	208,109,294
Common stock, shares outstanding	207,802,582	206,535,955
Common stock, shares unvested	938	1,126
Treasury stock, shares	1,573,339	1,573,339
Class A Non-Voting Common Stock [Member]
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	5,000,000	5,000,000
Common stock, shares issued	0	0