Accounting Policies (Details)	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)		Dec. 31, 2017 USD ($)		Dec. 31, 2016 USD ($)		Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Oct. 01, 2010 USD ($)	Sep. 30, 2010 USD ($)
Disaggregation of Revenue [Line Items]
Revenues(1)	$ 966,756,000	$ 925,126,000	$ 922,295,000	$ 1,014,372,000	$ 826,494,000	$ 795,513,000	$ 795,552,000	$ 882,664,000	$ 3,828,549,000	[1]	$ 3,300,223,000	[1]	$ 2,941,347,000	[1]
Segment Reporting [Abstract]
Number of reportable segments (integer)									1
Property, Plant and Equipment [Line Items]
Land															$ 6,215,000	$ 6,215,000
Buildings and Improvements, Gross															112,308,000	95,615,000
Computer hardware and software															115,259,000	105,060,000
Software development costs															71,332,000	60,568,000
Property, Plant and Equipment, Gross															350,808,000	310,349,000
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment															(233,595,000)	(214,690,000)
Furniture and Fixtures, Gross															45,694,000	42,891,000
Property, Plant and Equipment, Net															117,213,000	95,659,000
Amortization of capitalized computer software costs									$ 6,000,000		4,100,000		3,000,000
Unamortized computer software costs															19,600,000	14,900,000
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill, Impairment Loss									$ 0
Goodwill, Gross															21,156,000	21,156,000
Goodwill, Impaired, Accumulated Impairment Loss															(8,470,000)	(8,470,000)
Goodwill															12,686,000	12,686,000
Health Insurance Costs [Abstract]
Number of days cash in advance of beginning of reporting quarter United establishes cash funding rates									90 days
Prepaid health insurance															9,000,000	9,000,000
Required deposit for health care costs															6,000,000
Amount which Plan Costs were less than the net premiums paid and owed															(6,300,000)
Portion of insurance costs that is less than the agreed upon surplus included in accrued health insurance costs a cnt liab															(15,300,000)
Premiums owed to United															(15,200,000)
Benefits Costs Incurred Related to Run-off															(1,300,000)	1,200,000	$ 5,100,000
Workers' Compensation Costs [Abstract]
Company's maximum economic burden for the first layer of claims per occurrence																			$ 1,000,000
Company's maximum aggregate economic burden for claims in excess of $1 million per policy year																		$ 6,000,000
Reduction in accrued workers' compensation costs for changes in estimated losses									$ 18,800,000		$ 16,300,000		10,900,000
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)									2.60%		1.60%
Incurred but not paid workers' compensation liabilities
Beginning balance				207,630,000				183,928,000	$ 207,630,000		$ 183,928,000
Accrued claims									72,066,000		68,194,000
Present value discount									(7,829,000)		(4,308,000)
Paid claims									(42,228,000)		(40,184,000)
Ending balance	229,639,000				207,630,000				229,639,000		207,630,000		183,928,000
Current portion of accrued claims															42,227,000	41,137,000
Long-term portion of accrued claims															187,412,000	166,493,000
Ending Balance	$ 229,639,000			$ 207,630,000	$ 207,630,000			$ 183,928,000	207,630,000		183,928,000		$ 183,928,000		229,639,000	207,630,000	$ 183,928,000
Current portion of workers' compensation administrative fees accrued															3,600,000	1,800,000
Undiscounted accrued workers' compensation costs									$ 247,400,000		$ 219,900,000
Time period incurred claims expected to be paid recorded as restricted cash									1 year
Time period incurred claims expected to be paid, included in deposits, a long-term asset									Greater than 1 year
Return Of Excess Claim Funds									$ 19,400,000
Restricted Cash and Cash Equivalents, Current															42,227,000	41,137,000
Deposits - workers' compensation															$ 166,474,000	$ 154,215,000
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
Stock-Based Compensation [Abstract]
Number of stock-based employee compensation plans															1
Vesting period for Initial grants to new members of the Board of Directors									0 years		3 years
Vesting period for officers and other management to be eligible for restricted stock grants, minimum									3 years
Percentage of annual grants issued to directors that are vested (in hundredths)									100.00%
Corporate Employees [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Percentage the entity matches of eligible corporate employees' contributions									100.00%		100.00%
Percentage of eligible compensation matched, Minimum									0.00%		0.00%		0.00%
Percentage of eligible compensation matched, maximum									6.00%		6.00%		6.00%
Matching contributions to the plan									$ 10,300,000		$ 8,700,000		$ 8,000,000
Worksite Employees [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Percentage of eligible compensation matched, Minimum									0.00%
Percentage of eligible compensation matched, maximum									6.00%
Matching contributions to the plan									$ 165,500,000		129,000,000		108,300,000
Buildings and Improvements [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment									5 years
Buildings and Improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment									30 years
Computer Hardware and Software and Acquired Technologies [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment									2 years
Computer Hardware and Software and Acquired Technologies [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment									5 years
Software Development Costs [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment									3 years
Software Development Costs [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment									5 years
Furniture, Fixtures and Other [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment									5 years
Furniture, Fixtures and Other [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment									7 years
Northeast [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)									$ 996,541,000		854,629,000		750,748,000
Southeast [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)									447,584,000		379,874,000		318,185,000
Central [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)									637,779,000		543,486,000		467,297,000
Southwest [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)									895,243,000		767,207,000		689,334,000
West [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)									797,942,000		702,619,000		664,308,000
Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)									$ 53,460,000		$ 52,408,000		$ 51,475,000

[1] (1) Revenues are comprised of gross billings less worksite employee (“WSEE”) payroll costs as follows: Year ended December 31,(in thousands)201820172016 Gross billings$23,830,731$20,173,812$17,932,857Less: WSEE payroll cost20,002,18216,873,58914,991,510Revenues$3,828,549$3,300,223$2,941,347