Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accounts receivable, allowance	$ 2,825	$ 2,542
Accumulated depreciation	$ 1,487,134	$ 1,516,664
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	50,000,000	50,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	119,096,000	118,674,000
Common stock, shares outstanding	119,096,000	118,674,000