Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts and Bilinig Adjustments
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of year		$ 8,093	$ 6,682	$ 4,404
Provisions		5,251	2,881	2,196
Valuation Allowances and Reserves, Additions for Charges to Other Accounts	[1]	6,799	7,907	8,290
Deductions	[2]	(10,234)	(9,377)	(8,208)
Balance at end of year		9,909	8,093	6,682
Allowance for Workers' Compensation and Medical Malpractice Loss Reserves
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of year		15,784	15,020	15,564
Provisions		2,908	2,693	2,117
Valuation Allowances and Reserves, Additions for Charges to Other Accounts	[1]	0	0	0
Deductions	[2]	(3,915)	(1,929)	(2,661)
Balance at end of year		$ 14,777	$ 15,784	$ 15,020

[1] Charges to other accounts include the provision for permanent placement fallouts and billing adjustments that have been deducted from revenues in the accompanying consolidated statements of operations and comprehensive income.

[2] Deductions from the allowance for doubtful accounts and billing adjustments include write-offs of uncollectible accounts receivable and permanent placements fallouts that have been charged against the allowance for doubtful accounts and billing adjustments. Deductions from workers’ compensation loss reserves include payments of claims and changes related to anticipated insurance and indemnification recoveries which have corresponding changes in the workers' compensation receivable balance in the accompanying consolidated balance sheets.