Accounting Policies (Details)		12 Months Ended
		Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Oct. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)	Oct. 01, 2010 USD ($)	Sep. 30, 2010 USD ($)
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Percentage of eligible compensation matched, maximum		3.00%
Disaggregation of Revenue [Line Items]
Revenues(1)	[1]	$ 4,973,070,000	$ 4,287,004,000	$ 4,314,804,000
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill, Impairment Loss		0	0	0
Goodwill, Gross		21,200,000
Goodwill, Impaired, Accumulated Impairment Loss		8,500,000
Goodwill		$ 12,700,000
Health Insurance Costs [Abstract]
Percentage of our health insurance coverage provided by United		87.00%
Number of days in advance of the beginning of a reporting quarter carrier establishes cash funding rates		90 days
Prepaid health insurance		$ 9,000,000	9,000,000
Required deposit for health care costs		6,500,000
Portion of insurance costs that is less than the agreed upon surplus included in accrued health insurance costs a cnt liab		(31,000,000)
Prepaid health insurance current and noncurrent		(22,000,000)
Premiums owed to United		12,600,000
Benefits Costs Incurred Related to Run-off		(4,900,000)	(200,000)	(2,300,000)
Impaired Long-Lived Assets Held and Used [Line Items]
Property, Plant and Equipment, Gross		512,995,000	495,078,000
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment		302,272,000	278,822,000
Property and equipment, net		210,723,000	216,256,000
Amortization of capitalized computer software costs		10,900,000	8,700,000	7,800,000
Unamortized computer software costs		$ 35,400,000	32,600,000
Segment Reporting [Abstract]
Number of reportable segments (integer)		1
Share-based Compensation Arrangement by Share-based Payment Award, Additional General Disclosures [Abstract]
Number of stock-based employee compensation plans		1
Stock awards vesting period		3 years
Percentage of grants issued to directors that are vested for annual award grants.		100.00%
Workers' Compensation Costs [Abstract]
Workers' Compensation, Maximum economic burden first layer of claims per occurrence					$ 1,500,000			$ 1,000,000
Workers' Compensation, Maximum aggregate economic burden for claims in excess of first layer cap per policy year					$ 6,000,000		$ 6,000,000
Decrease Increase in accrued workers' compensation costs for changes in estimated losses		$ 41,700,000	42,100,000	31,700,000
U.S. Treasury rates that correspond with the weighted average estimated claim payout period		0.60%
Restricted Cash and Cash Equivalents, Current		$ 46,929,000	45,522,000	49,295,000		$ 42,227,000
Deposits workers' compensation		185,027,000	186,331,000	175,913,000		166,474,000
Incurred but not paid workers' compensation liabilities
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustments			240,761,000	242,904,000
Workers' Compensation Expense		48,097,000	42,639,000
Workers' Compensation Discount, Changed during period		898,000	(740,000)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid		(48,337,000)	(45,522,000)
Ending Balance		239,623,000	240,761,000
Current portion of accrued claims		46,929,000	45,522,000
Long-term portion of accrued claims		192,694,000	195,239,000
Workers' compensation administrative fees accrued		3,600,000	2,700,000
Undiscounted accrued workers' compensation costs		$ 255,100,000	257,900,000
Time period incurred claims expected to be paid recorded as restricted cash		1 year
Return Of Excess Claim Funds		$ 35,100,000
Restricted Cash and Cash Equivalents, Current		46,929,000	45,522,000	49,295,000		42,227,000
Deposits workers' compensation		$ 185,027,000	$ 186,331,000	$ 175,913,000		$ 166,474,000
Time period estimate of incurred claim costs to be paid included in short term liabilities		1 year
Minimum [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Estimated useful lives of property and equipment		5 years	7 years
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%	6.00%	6.00%
Matching contributions to the plan		$ 8,200,000	$ 12,600,000	$ 11,400,000
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		$ 244,100,000	199,200,000	198,200,000
Land [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Property, Plant and Equipment, Gross		6,215,000	6,215,000
Building and Building Improvements [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Property, Plant and Equipment, Gross		$ 206,449,000	$ 125,704,000
Building and Building Improvements [Member] | Minimum [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Estimated useful lives of property and equipment		5 years
Building and Building Improvements [Member] | Maximum [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Estimated useful lives of property and equipment			30 years
Computer hardware and software [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Property, Plant and Equipment, Gross		$ 136,346,000	$ 129,213,000
Computer hardware and software [Member] | Minimum [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Estimated useful lives of property and equipment		2 years
Computer hardware and software [Member] | Maximum [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Estimated useful lives of property and equipment			5 years
Software Development Costs [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Property, Plant and Equipment, Gross		$ 112,433,000	$ 100,762,000
Software Development Costs [Member] | Minimum [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Estimated useful lives of property and equipment		3 years
Software Development Costs [Member] | Maximum [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Estimated useful lives of property and equipment			5 years
Construction in Progress [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Property, Plant and Equipment, Gross		$ 0	$ 84,668,000
Furniture, Fixtures and Other [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Property, Plant and Equipment, Gross		51,552,000	48,516,000
Northeast [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)		1,390,156,000	1,189,837,000	1,135,771,000
Southeast [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)		630,342,000	509,846,000	499,201,000
Central [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)		867,914,000	761,905,000	743,514,000
Southwest [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)		993,747,000	935,634,000	1,001,845,000
West [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)		1,033,996,000	839,347,000	880,434,000
Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)		$ 56,915,000	$ 50,435,000	$ 54,039,000

[1] Revenues are comprised of gross billings less worksite employee (“WSEE”) payroll costs as follows: Year ended December 31, (in thousands) 202120202019 Gross billings $33,318,693 $28,168,611 $27,212,010  Less: WSEE payroll cost 28,345,623 23,881,607 22,897,206  Revenues $4,973,070 $4,287,004 $4,314,804