Long-Term Debt and Credit Facility - Additional Information (Details)		6 Months Ended
	Feb. 27, 2018	Jun. 30, 2018 USD ($)	Mar. 12, 2018 USD ($)	Dec. 31, 2017 USD ($)	Oct. 31, 2017 USD ($)	Oct. 31, 2015 USD ($)
Debt Instrument [Line Items]
Fees		$ 2,500,000
Up front lending fees		900,000
Third party arrangement fees		1,600,000
Third-party notes and bank debt held		800,000		$ 900,000
Debt designated as third-party debt		7,800,000		7,800,000
Long-term debt, fair value		342,900,000		356,000,000
2015 Interest Rate Swap
Debt Instrument [Line Items]
Interest rate at period end						3.52%
Notional amount						$ 262,500,000
Fixed interest rate						1.46%
2018 Interest Rate Swap
Debt Instrument [Line Items]
Notional amount			$ 170,600,000
Portion of amortization of term loan			$ 170,600,000
Fixed interest rate			2.937%
Term Loan [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity		308,400,000.0
2015 Credit Facility
Debt Instrument [Line Items]
Senior secured credit facility						$ 650,000,000
Maximum borrowing capacity					$ 350,000,000
Secured debt outstanding		295,300,000		308,400,000
Proceeds from line of credit, net		47,000,000
Letters of credit issued and outstanding		$ 31,000,000
Line of credit				$ 38,000,000
Portion of amortization of term loan						$ 262,500,000
Initial maximum financial leverage ratio		3.75
Actual financial leverage ratio		3.29
Remaining borrowing capacity		$ 49,400,000
Minimum permitted fixed charge coverage ratio		1.15
Actual fixed charge coverage ratio		1.34
2015 Credit Facility | Insurance Carriers Collateral
Debt Instrument [Line Items]
Letters of credit issued and outstanding		$ 13,500,000
2015 Credit Facility | Work Performance Obligations
Debt Instrument [Line Items]
Letters of credit issued and outstanding		$ 17,500,000
2015 Credit Facility | LIBOR | 2015 Interest Rate Swap
Debt Instrument [Line Items]
Interest rate at period end		4.08%
2015 Credit Facility | LIBOR | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate		1.25%
2015 Credit Facility | LIBOR | Maximum
Debt Instrument [Line Items]
Basis spread on variable rate		2.25%
2015 Credit Facility | Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Term		5 years
2015 Credit Facility | Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 300,000,000.0
Term of revolving line of credit	5 years
Maximum borrowing capacity, post sale		275,000,000.0
Unamortized loan costs		2,400,000
Interest expense, debt		200,000
Proceeds from line of credit, net		$ 9,000,000
Line of Credit
Debt Instrument [Line Items]
Interest rate at period end		4.08%		3.50%
Line of credit		$ 47,000,000		$ 38,000,000
2018 Credit Facility
Debt Instrument [Line Items]
Notional amount			$ 170,600,000