CONSOLIDATED STATEMENTS OF OPERATIONS (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Gross billings	$ 23,830,731	$ 20,173,812	$ 17,932,857
Worksite employee payroll costs	$ 20,002,182	$ 16,873,589	$ 14,991,510