CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Preferred stock, per share stated value	$ 0.000025	$ 0.000025
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, per share stated value	$ 0.000025	$ 0.000025
Common stock, shares authorized	750,000,000	750,000,000
Common stock, shares issued	69,429,088	69,015,690
Common stock, shares outstanding	69,429,088	69,015,690
Collateral paid to insurance carriers, net of workers' compensation loss reserves, noncurrent	$ 22,911	$ 22,377