Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Allowance for doubtful accounts receivable	$ 2,241	$ 2,500
Accumulated amortization of finite-lived intangible assets	$ 416,708	$ 338,888
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	82,268,722	80,478,620
Common stock, shares outstanding	81,482,283	79,966,907
Common stock, shares unvested	17,188	25,000
Treasury stock, shares	786,000	511,000
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	2,703,846	2,703,846
Common stock, shares outstanding	2,703,846	2,703,846