Long-Term Debt, Net (Details)		12 Months Ended
	Jun. 19, 2018 USD ($)	Dec. 31, 2018 USD ($) quarter	Dec. 31, 2011 USD ($) series	Jun. 18, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2014 instrument
Debt Instrument [Line Items]
Long-term Debt		$ 292,000,000			$ 228,000,000
Long-term Debt, Net		$ 289,770,000			226,989,000
Number of series of debt issued | series			2
Maturity date		Jun. 19, 2023
Previous borrowing capacity				$ 400,000,000
Current borrowing capacity	$ 300,000,000
Uncommitted borrowings available	100,000,000
Maximum borrowing capacity	$ 400,000,000
Basis spread on variable rate		1.375%
Performance bonds under credit facility		$ 18,000,000
Remaining borrowing capacity		140,000,000
Performance bonds		13,300,000
Debt Issuance Costs, Net		2,230,000			1,011,000
Interest Rate Swap [Member]
Debt Instrument [Line Items]
Derivative, number of instruments held | instrument						2
Senior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt		150,000,000			150,000,000
Face amount			$ 150,000,000
Credit Facility [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 142,000,000			$ 78,000,000
Number of quarters used for measurement of covenants | quarter		4
Interest coverage ratio, minimum		3.0
Covenant leverage ratio, minimum		2.5
Senior Notes Series A [Member]
Debt Instrument [Line Items]
Face amount			$ 75,000,000
Interest rate, effective percentage			4.01%
Maturity date			Sep. 30, 2021
Senior Notes Series B [Member]
Debt Instrument [Line Items]
Face amount			$ 75,000,000
Interest rate, effective percentage			4.11%
Maturity date			Sep. 30, 2023
Maximum [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	2.00%