Long-Term Debt and Credit Facility (Details)		9 Months Ended
	Mar. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)	Jul. 31, 2013 USD ($)
Debt Instrument [Line Items]
Actual financial leverage ratio		2.97
Proceeds from line of credit		$ 26,000,000	$ 10,000,000
Other Long-term Debt		11,600,000		$ 12,400,000
Other Loans Payable		100,000		100,000
Letters of credit issued and outstanding		37,200,000
Long-term debt, fair value		$ 357,000,000		377,000,000
Initial maximum financial leverage ratio		3.25
Remaining borrowing capacity		$ 34,400,000
Minimum permitted fixed charge coverage ratio		1.25
Actual fixed charge coverage ratio		1.34
Insurance Carriers Collateral [Member]
Debt Instrument [Line Items]
Letters of credit issued and outstanding		$ 16,400,000
Work Performance Obligations [Member]
Debt Instrument [Line Items]
Letters of credit issued and outstanding		$ 20,800,000
2013 Interest Rate Swap [Member]
Debt Instrument [Line Items]
Notional Amount					$ 175,000,000
Fixed interest rate					0.87%
Annualized borrowing rate		2.87%
2013 Credit Facility [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount					$ 650,000,000
Maximum borrowing capacity					300,000,000
Term of credit facility		5 years
Secured Debt		$ 273,200,000		319,400,000	350,000,000
Debt Instrument, Term		5 years
Amount drawn on credit facility		$ 71,500,000		$ 45,500,000
Annual mandatory prepayment, percent of Excess Cash Flow		50.00%
Minimum Consolidated Leverage Ratio required		2.50
Actual financial leverage ratio				2.90
Term loan repayment	$ 26,500,000
Proceeds from line of credit	$ 26,000,000
Derivative, Amount of Hedged Item					$ 175,000,000
2013 Credit Facility [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Interest rate at period end		2.06%
2013 Credit Facility [Member] | LIBOR [Member] | Minimum [Member]
Debt Instrument [Line Items]
Basis spread on variable rate		1.25%
2013 Credit Facility [Member] | LIBOR [Member] | Maximum [Member]
Debt Instrument [Line Items]
Basis spread on variable rate		2.25%