Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Current assets:
Trade accounts receivable, allowance for doubtful accounts	$ 11,123	$ 11,729
Preferred stock
Par value (in dollars per share)	$ 0.01	$ 0.01
Authorized (in shares)	5,000,000	5,000,000
Issued (in shares)	0	0
Outstanding (in shares)	0	0
Common stock
Par value (in dollars per share)	$ 0.01	$ 0.01
Authorized (in shares)	400,000,000	400,000,000
Issued (in shares)	288,945,899	289,581,682
Outstanding (in shares)	288,945,899	289,581,682