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Commitments and Contingencies (Details)
Jun. 30, 2018
USD ($)
letter_of_credit
Debt Instrument [Line Items]  
Number of letters of credit provided as collateral | letter_of_credit 2
Standby Letters of Credit  
Debt Instrument [Line Items]  
Current borrowing capacity $ 18,000,000
Credit Facility | Revolving Credit Facility  
Debt Instrument [Line Items]  
Maximum borrowing capacity under credit facility 250,000,000.0
Unused portion of credit facility 234,000,000
Credit Facility | Swingline Facility  
Debt Instrument [Line Items]  
Maximum borrowing capacity under credit facility 20,000,000
Collateral for Workers' Compensation Obligation | Line of Credit  
Debt Instrument [Line Items]  
Maximum borrowing capacity under credit facility 5,000,000.0
Unused portion of credit facility $ 3,000,000