Long-Term Debt and Credit Facility - Additional Information (Details)	1 Months Ended	9 Months Ended
	Oct. 31, 2015 USD ($)	Sep. 30, 2017 USD ($)	Jul. 01, 2017	Jun. 30, 2017	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Amount retired under prior credit facility	$ 344,700,000
Third-party notes and bank debt held		$ 400,000			$ 300,000
Debt designated as third-party debt		7,700,000			9,600,000
Long-term debt, fair value		$ 373,900,000			366,000,000
2015 Interest Rate Swap
Debt Instrument [Line Items]
Notional amount	$ 262,500,000
Fixed interest rate	1.46%
2015 Credit Facility
Debt Instrument [Line Items]
Senior secured credit facility	$ 650,000,000
Maximum borrowing capacity	$ 350,000,000
Term of revolving line of credit	5 years
Term of term loan facility	5 years
Interest rate at period end		3.26%
Secured debt outstanding		$ 315,000,000			328,100,000
Proceeds from line of credit, net		50,000,000
Letters of credit issued and outstanding		$ 38,200,000
Line of credit					$ 36,000,000
Portion of amortization of term loan	$ 262,500,000
Initial maximum financial leverage ratio			3.50	3.75
Actual financial leverage ratio		2.98
Remaining borrowing capacity		$ 65,900,000
Minimum permitted fixed charge coverage ratio		1.25
Actual fixed charge coverage ratio		1.90
2015 Credit Facility | Insurance Carriers Collateral
Debt Instrument [Line Items]
Letters of credit issued and outstanding		$ 16,200,000
2015 Credit Facility | Work Performance Obligations
Debt Instrument [Line Items]
Letters of credit issued and outstanding		$ 22,000,000
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 | Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 300,000,000
Proceeds from line of credit, net		$ 14,000,000
Line of Credit
Debt Instrument [Line Items]
Interest rate at period end		3.23%			2.96%
Line of credit		$ 50,000,000			$ 36,000,000