Accounting Policies (Details)		12 Months Ended
		Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Oct. 01, 2019 USD ($)	Sep. 30, 2019 USD ($)
Disaggregation of Revenue [Line Items]
Revenues(1)	[1]	$ 5,938,818,000	$ 4,973,070,000	$ 4,287,004,000
Segment Reporting [Abstract]
Number of reportable segments (integer)		1
Impaired Long-Lived Assets Held and Used [Line Items]
Property, Plant and Equipment, Gross		$ 530,613,000	512,995,000
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment		330,621,000	302,272,000
Property and equipment, net		199,992,000	210,723,000
Amortization of capitalized computer software costs		13,200,000	10,900,000	8,700,000
Unamortized computer software costs		33,700,000	35,400,000
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill, Impairment Loss		$ 0	0	0
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		(5,300,000)
Prepaid health insurance current and noncurrent		3,700,000
Premiums owed to United		46,400,000
Benefits Costs Incurred Related to Run-off		(12,100,000)	4,900,000	200,000
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		$ 42,200,000	$ 41,700,000	42,100,000
U.S. Treasury rates that correspond with the weighted average estimated claim payout period		2.90%	0.60%
Incurred but not paid workers' compensation liabilities
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustments			$ 239,623,000	240,761,000
Workers' Compensation Expense		$ 49,121,000	48,097,000
Workers' Compensation Discount, Changed during period		9,517,000	898,000
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid		(49,819,000)	(48,337,000)
Ending Balance		229,408,000	239,623,000
Current portion of accrued claims		49,779,000	46,929,000
Long-term portion of accrued claims		179,629,000	192,694,000
Workers' compensation administrative fees accrued		3,700,000	3,600,000
Undiscounted accrued workers' compensation costs		$ 250,500,000	255,100,000
Time period incurred claims expected to be paid recorded as restricted cash		1 year
Return Of Excess Claim Funds		$ 30,200,000
Restricted Cash and Cash Equivalents, Current		49,779,000	46,929,000	45,522,000	$ 49,295,000
Deposits workers' compensation		$ 196,370,000	$ 185,027,000	$ 186,331,000	$ 175,913,000
Time period estimate of incurred claim costs to be paid included in short term liabilities		1 year
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%
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Percentage of eligible compensation matched, maximum		3.00%
Minimum [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Estimated useful lives of property and equipment		5 years
Maximum [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Estimated useful lives of property and equipment		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		$ 14,400,000	$ 8,200,000	$ 12,600,000
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		$ 328,500,000	244,100,000	199,200,000
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		$ 207,740,000	206,449,000
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		$ 141,856,000	136,346,000
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		$ 123,967,000	112,433,000
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		3 years
Furniture, Fixtures and Other [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Property, Plant and Equipment, Gross		$ 50,835,000	51,552,000
Northeast [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)		1,624,556,000	1,390,156,000	1,189,837,000
Southeast [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)		796,219,000	630,342,000	509,846,000
Central [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)		1,045,043,000	867,914,000	761,905,000
Southwest [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)		1,163,088,000	993,747,000	935,634,000
West [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)		1,251,186,000	1,033,996,000	839,347,000
Other Revenues [Member]
Disaggregation of Revenue [Line Items]
Revenues(1)		$ 58,726,000	$ 56,915,000	$ 50,435,000

[1] Revenues are comprised of gross billings less worksite employee (“WSEE”) payroll costs as follows: Year ended December 31, (in thousands) 202220212020 Gross billings $40,126,910 $33,318,693 $28,168,611  Less: WSEE payroll cost 34,188,092 28,345,623 23,881,607  Revenues $5,938,818 $4,973,070 $4,287,004