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Debt - Narrative (Details)			3 Months Ended			9 Months Ended		12 Months Ended
Debt - Narrative (Details)	Sep. 30, 2020 USD ($) entity	Oct. 02, 2019 USD ($)	Sep. 30, 2020 USD ($) entity	Jun. 30, 2020 USD ($) entity loan	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) entity	Sep. 30, 2019 USD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 05, 2019 USD ($)
Debt Instrument [Line Items]
Long-term debt	$ 400,429,000		$ 400,429,000			$ 400,429,000							$ 443,035,000
Amortization of debt issuance costs			727,000		$ 0	2,219,000	$ 1,555,000
Repayments of debt						52,695,000	$ 10,435,000
Finance lease liabilities	$ 610,000		$ 610,000			$ 610,000							764,000
Number of entities \| entity	2		2	2		2
Ownership interest in subsidiaries	50.00%		50.00%	50.00%		50.00%
Paycheck protection loans received, CARES Act \| loan				3
Paycheck protection program loan amount, CARES Act				$ 9,800,000
Plateau Excavation
Debt Instrument [Line Items]
Liabilities incurred		$ 10,000,000
Tealstone Construction
Debt Instrument [Line Items]
Repayment of deferred cash payment						$ 5,000,000
Secured Debt
Debt Instrument [Line Items]
Fixed charge ratio minimum						1.20
Secured Debt \| Credit Agreement
Debt Instrument [Line Items]
Debt instrument face amount		475,000,000
Secured Debt \| Credit Facility
Debt Instrument [Line Items]
Long-term debt	$ 380,000,000		$ 380,000,000			$ 380,000,000							420,000,000
Secured Debt \| Term Loan Facility
Debt Instrument [Line Items]
Long-term debt	$ 380,000,000		$ 380,000,000			$ 380,000,000							400,000,000
Weighted average interest rate	5.80%		5.80%			5.80%
Notes Payable, Other Payables \| Plateau Excavation
Debt Instrument [Line Items]
Liabilities incurred		$ 10,000,000
Stated interest rate		8.00%
Notes Payable, Other Payables \| Tealstone Construction
Debt Instrument [Line Items]
Long-term debt	$ 0		$ 0			$ 0							$ 12,230,000
Repayments of debt						$ 7,500,000
Notes Payable, Other Payables \| Notes payable for transportation and construction equipment and other \| Minimum
Debt Instrument [Line Items]
Stated interest rate	2.99%		2.99%			2.99%
Debt instrument, term						3 years
Notes Payable, Other Payables \| Notes payable for transportation and construction equipment and other \| Maximum
Debt Instrument [Line Items]
Stated interest rate	6.92%		6.92%			6.92%
Debt instrument, term						5 years
Revolving Credit Facility \| Secured Debt \| Term Loan Facility
Debt Instrument [Line Items]
Debt instrument face amount		$ 400,000,000
Revolving Credit Facility \| Line of Credit \| Revolving Credit Facility
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity		75,000,000
Debt instrument, base interest rate	4.25%
Debt instrument, LIBOR interest rate	0.162%
Line of credit, remaining borrowing capacity	$ 75,000,000		$ 75,000,000			$ 75,000,000
Weighted average interest rate	6.68%		6.68%			6.68%
Revolving Credit Facility \| Line of Credit \| Revolving Credit Facility \| Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate	3.50%
Revolving Credit Facility \| Line of Credit \| Revolving Credit Facility \| London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate	4.50%
Swing Line Loan \| Line of Credit \| Revolving Credit Facility
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity		$ 15,000,000
Interest Rate Swap \| Secured Debt
Debt Instrument [Line Items]
Derivative, notional amount	$ 350,000,000		$ 350,000,000			$ 350,000,000								$ 350,000,000
Forecast \| Secured Debt \| Term Loan Facility
Debt Instrument [Line Items]
Periodic payments								$ 220,000,000	$ 50,000,000	$ 50,000,000	$ 50,000,000	$ 30,000,000
October 1, 2019 - June 30, 2020 \| Secured Debt \| Credit Agreement
Debt Instrument [Line Items]
Leverage ratio maximum						4.00
July 1, 2020 - September 30, 2020 \| Secured Debt \| Credit Agreement
Debt Instrument [Line Items]
Leverage ratio maximum						3.75
October 1, 2020 - March 31, 2021 \| Secured Debt \| Credit Agreement
Debt Instrument [Line Items]
Leverage ratio maximum						3.50
April 1, 2021 - June 30, 2021 \| Secured Debt \| Credit Agreement
Debt Instrument [Line Items]
Leverage ratio maximum						3.25
October 1, 2021 - December 31, 2021 \| Secured Debt \| Credit Agreement
Debt Instrument [Line Items]
Leverage ratio maximum						3.00