Commitments and Contingencies (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) property	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Other Commitments [Line Items]
Self Insurance Reserve	$ 2,100	$ 1,800
Operating Leases, Rent Expense	26,400	25,600	$ 21,600
Deferred Rent Credit, Current	1,200	600
Deferred Rent Credit, Noncurrent	5,000	5,200
Workerscompensationreceivable	12,702	14,001
Letters of Credit Outstanding, Amount	4,400	4,000
Commitment and contingencies [Line Items]
Costs and Expenses, Related Party	1,300	$ 1,300	$ 1,300
Operating Leases
2018	20,387
2019	17,520
2020	14,698
2021	12,146
2022	8,089
Thereafter	7,267
Total	80,107
Related Party
2018	1,302
2019	1,282
2020	1,314
2021	1,347
2022	1,380
Thereafter	2,618
Total	9,243
Purchase Obligation, Due in Next Twelve Months	10,283
Purchase Obligation, Due in Second Year	8,362
Purchase Obligation, Due in Third Year	1,055
Purchase Obligation, Due in Fourth Year	0
Purchase Obligation, Due in Fifth Year	0
Purchase Obligation, Due after Fifth Year	0
Purchase Obligation	19,700
Total
2018	31,972
2019	27,164
2020	17,067
2021	13,493
2022	9,469
Thereafter	9,885
Total	$ 109,050
Apex Systems
Commitment and contingencies [Line Items]
Number of Leased Properties Owned by Related Parties | property	2