Long-Term Debt (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2014 USD ($) interest_rate_swap
Debt Instrument [Line Items]
Long-term Debt	$ 236,000	$ 228,000
Less: Debt issuance costs	(886)	(1,011)
Long-term debt, net	235,114	226,989
Line of credit facility, current borrowing capacity	400,000
Line of Credit Facility, Extension Capacity	50,000
Line of credit facility, maximum borrowing capacity	450,000
Performance bonds under credit facility	12,300
Line of credit facility, remaining borrowing capacity	301,700
Performance bonds	$ 13,400
Number of interest rate swaps | interest_rate_swap			2
Minimum Interest Coverage Ratio	300.00%
Maximum Leverage Ratio	250.00%
Senior Notes
Debt Instrument [Line Items]
Long-term Debt	$ 150,000	150,000
Face amount	150,000
Line of Credit
Debt Instrument [Line Items]
Long-term Debt	$ 86,000	$ 78,000
Maturity date	Aug. 29, 2019
Senior Notes Series A
Debt Instrument [Line Items]
Face amount	$ 75,000
Interest rate, effective percentage	4.01%
Maturity date	Sep. 30, 2021
Senior Notes Series B
Debt Instrument [Line Items]
Face amount	$ 75,000
Interest rate, effective percentage	4.11%
Maturity date	Sep. 30, 2023
Interest Rate Swap
Debt Instrument [Line Items]
Notional amount			$ 50,000