Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Allowance for doubtful accounts	$ 15,658	$ 17,154
Common stock, shares issued	263,498,672	263,660,803
Common stock, shares outstanding	263,362,026	263,494,670
Treasury shares	136,646	166,133