Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Accounts receivable, credit loss reserve	$ 6,777	$ 6,103
Accumulated depreciation	$ 1,602,276	$ 1,559,388
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	120,285,000	119,638,000
Common stock, shares outstanding	120,285,000	119,638,000