Accounting Policies (Details)	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)		Dec. 31, 2018 USD ($)		Dec. 31, 2017 USD ($)		Dec. 31, 2019 USD ($)	Oct. 01, 2019 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2016 USD ($)	Oct. 01, 2010 USD ($)	Sep. 30, 2010 USD ($)
Disaggregation of Revenue [Line Items]
Revenues(1)	$ 1,075,090,000	$ 1,043,388,000	$ 1,043,316,000	$ 1,153,010,000	$ 966,756,000	$ 925,126,000	$ 922,295,000	$ 1,014,372,000	$ 4,314,804,000	[1]	$ 3,828,549,000	[1]	$ 3,300,223,000	[1]
Segment Reporting [Abstract]
Number of reportable segments (integer)									1
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross															$ 403,301,000			$ 350,808,000
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment															(255,595,000)			(233,595,000)
Property, Plant and Equipment, Net															147,706,000			117,213,000
Amortization of capitalized computer software costs									$ 7,800,000		6,000,000.0		4,100,000
Unamortized computer software costs															25,800,000			19,600,000
Leases [Abstract]
Right-of-use leased assets															56,886,000		$ 50,800,000	0
Operating Lease, Liability															74,006,000		$ 63,700,000
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill, Impairment Loss									$ 0
Goodwill, Gross															21,200,000
Goodwill, Impaired, Accumulated Impairment Loss															(8,500,000)
Goodwill															12,700,000
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															9,000,000.0			9,000,000
Required deposit for health care costs															6,500,000
Amount which Plan Costs were less than the net premiums paid and owed															2,600,000
Portion of insurance costs that is less than the agreed upon surplus included in accrued health insurance costs a cnt liab															(6,400,000)
Premiums owed to United															(9,300,000)
Benefits Costs Incurred Related to Run-off													1,200,000		2,300,000			(1,300,000)
Workers' Compensation Costs [Abstract]
Company's maximum economic burden for the first layer of claims per occurrence																$ 1,500,000					$ 1,000,000
Company's maximum aggregate economic burden for claims in excess of $1 million per policy year																$ 6,000,000				$ 6,000,000
Reduction in accrued workers' compensation costs for changes in estimated losses									$ 31,700,000		$ 18,800,000		16,300,000
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths)									1.90%		2.60%
Incurred but not paid workers' compensation liabilities
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustments													207,630,000					229,639,000
Accrued claims									$ 63,538,000		$ 72,066,000
Present value discount									(4,629,000)		(7,829,000)
Paid claims									(45,644,000)		(42,228,000)
Ending balance	242,904,000				229,639,000				242,904,000		229,639,000
Current portion of accrued claims															49,295,000			42,227,000
Long-term portion of accrued claims															193,609,000			187,412,000
Ending Balance	$ 242,904,000				$ 229,639,000				242,904,000		229,639,000				242,904,000			229,639,000
Current portion of workers' compensation administrative fees accrued															3,600,000			3,600,000
Undiscounted accrued workers' compensation costs									$ 262,900,000		247,400,000
Time period incurred claims expected to be paid recorded as restricted cash									1 year
Return Of Excess Claim Funds									$ 18,800,000
Deposits workers' compensation													154,215,000		$ 175,913,000			166,474,000	$ 143,938,000
Time period estimate of incurred claim costs to be paid included in short term liabilities									1 year
Stock-Based Compensation [Abstract]
Number of stock-based employee compensation plans															1
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%
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Percentage of our health insurance coverage provided by United															87.00%
Corporate Employees [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Percentage the entity matches of eligible corporate employees' contributions									100.00%
Percentage of eligible compensation matched, maximum									6.00%
Matching contributions to the plan									$ 11,400,000		10,300,000		8,700,000
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									$ 198,200,000		165,500,000		129,000,000.0
Land [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross															$ 6,215,000			6,215,000
Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross															119,635,000			112,308,000
Building and Building Improvements [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment									5 years
Building and Building Improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment									30 years
Computer hardware and software [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross															123,232,000			115,259,000
Computer hardware and software [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment									2 years
Computer hardware and software [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives of property and equipment									5 years
Software Development Costs [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross															85,252,000			71,332,000
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									3 years
Furniture, Fixtures and Other [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross															48,082,000			45,694,000
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
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross															$ 20,885,000			$ 0
Northeast [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)									$ 1,135,771,000		996,541,000		854,629,000
Southeast [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)									499,201,000		447,584,000		379,874,000
Central [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)									743,514,000		637,779,000		543,486,000
Southwest [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)									1,001,845,000		895,243,000		767,207,000
West [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)									880,434,000		797,942,000		702,619,000
Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)									$ 54,039,000		$ 53,460,000		$ 52,408,000

[1] ⁽¹⁾  Revenues are comprised of gross billings less worksite employee (“WSEE”) payroll costs as follows:   Year ended December 31, (in thousands) 2019 2018 2017         Gross billings $ 27,212,010
$ 23,830,731
$ 20,173,812
Less: WSEE payroll cost 22,897,206
20,002,182
16,873,589
Revenues $ 4,314,804
$ 3,828,549
$ 3,300,223