Long-Term Debt (Details)		9 Months Ended
	Jun. 19, 2018 USD ($)	Sep. 30, 2018 USD ($) interest_rate_swap	Dec. 31, 2017 USD ($)	Dec. 31, 2014 USD ($) interest_rate_swap
Debt Instrument [Line Items]
Long-term debt		$ 298,000,000	$ 228,000,000
Less: Debt issuance costs		(2,255,000)	(1,011,000)
Long-term debt, net		295,745,000	226,989,000
Line of credit facility, prior borrowing capacity	$ 400,000,000
Line of credit facility, current borrowing capacity	300,000,000
Line of credit facility, extension capacity	100,000,000
Line of credit facility, maximum borrowing capacity	$ 400,000,000
Performance bonds under credit facility		16,500,000
Line of credit facility, remaining borrowing capacity		135,500,000
Performance bonds		$ 13,900,000
Minimum interest coverage ratio		300.00%
Maximum leverage ratio		250.00%
Number of interest rate swaps | interest_rate_swap		2		2
Senior Notes
Debt Instrument [Line Items]
Long-term debt		$ 150,000,000	150,000,000
Face amount		150,000,000
Line of Credit
Debt Instrument [Line Items]
Long-term debt		148,000,000	$ 78,000,000
Maturity date	Jun. 19, 2023
Senior Notes Series A
Debt Instrument [Line Items]
Face amount		$ 75,000,000
Interest rate, effective percentage		4.01%
Maturity date		Sep. 30, 2021
Senior Notes Series B
Debt Instrument [Line Items]
Face amount		$ 75,000,000
Interest rate, effective percentage		4.11%
Maturity date		Sep. 30, 2023
Interest Rate Swap
Debt Instrument [Line Items]
Notional amount		$ 50,000,000		$ 50,000,000
London Interbank Offered Rate (LIBOR) | Minimum
Debt Instrument [Line Items]
Derivative, basis spread on variable rate		1.375%
London Interbank Offered Rate (LIBOR) | Maximum
Debt Instrument [Line Items]
Derivative, basis spread on variable rate		2.00%