Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2018		Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Oct. 01, 2010
Health Insurance Costs [Abstract]
Number of days in advance of the beginning of a reporting quarter United establishes cash funding rates				90 days
Required accumulated cash surplus	$ 9,000			$ 9,000		$ 9,000
Required deposit equal to approximately one day of claims funding activity	5,100			5,100
Amount which plan costs were less than the net premiums paid and owed	18,900			18,900
Prepaid health insurance, current	9,900			9,900
Premiums owed to United	28,500			28,500
Benefits costs incurred (reduced) related to run-off	(1,300)		$ (1,000)	(1,300)	$ (1,000)
Workers' Compensation Costs [Abstract]
Company's maximum economic burden for the first layer of claims per occurrence								$ 1,000
Company's maximum aggregate economic burden for claims in excess of 1 million per policy year								$ 5,000
Decrease Increase in accrued workers' compensation costs for changes in estimated losses				$ (13,400)	$ (11,000)
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)				2.50%	1.50%
Incurred but not paid workers compensation liabilities [Abstract]
Beginning balance, January 1,				$ 207,630	$ 183,928
Accrued claims				54,744	52,133
Present value discount				(5,372)	(3,048)
Paid claims				(31,645)	(29,574)
Ending balance	225,357		203,439	225,357	203,439
Current portion of accrued claims	42,258		41,748	42,258	41,748
Long-term portion of accrued claims	183,099		161,691	183,099	161,691	166,493
Workers compensation administrative fees accrued	3,500			3,500
Undiscounted accrued workers' compensation costs				$ 241,300	215,000
Time period incurred claims expected to be paid recorded as restricted cash				1 year
Return Of Excess Claim Funds				$ 19,400	22,700
Time period incurred claims expected to be paid, included in deposits, a long-term asset				Greater than 1 year
Restricted cash	42,257		41,748	$ 42,257	41,748	41,137	$ 42,637
Deposits workers compensation	152,268		142,821	$ 152,268	142,821	$ 154,215	$ 143,938
Time period estimate of incurred claim costs to be paid included in short-term liabilities				1 year
Time period incurred claims expected to be paid, included in long-term liabilities				Greater than 1 year
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	925,126	[1]	795,513	$ 2,861,793	2,473,729
Northeast [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	237,076		204,318	748,717	644,703
Southeast [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	109,552		92,646	333,004	283,501
Central [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	155,184		131,978	476,277	405,854
Southwest [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	217,946		184,572	664,731	573,773
West [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	192,011		169,321	599,617	527,217
Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 13,357		$ 12,678	$ 39,447	$ 38,681

[1] ____________________________________(1) Revenues are comprised of gross billings less worksite employee (“WSEE”) payroll costs as follows: Three Months Ended September 30, Nine Months Ended September 30,(in thousands)20182017 20182017 Gross billings$5,810,779$4,898,333 $17,284,477$14,655,545Less: WSEE payroll cost4,885,6534,102,820 14,422,68412,181,816Revenues$925,126$795,513 $2,861,793$2,473,729