CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accounts receivable, allowances	$ 13,370	$ 14,542
Accumulated depreciation and amortization	$ 944,455	$ 945,401
Common stock, par value (in usd per share)	$ 0.1	$ 0.1
Class A Common Stock
Authorized shares (in shares)	300,000,000	300,000,000
Issued shares (in shares)	172,998,668	170,276,449
Treasury shares (in shares)	8,870,801	8,870,801
Class B Common Stock
Authorized shares (in shares)	803,408	803,763
Issued shares (in shares)	803,408	803,763