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Organization, Business and Significant Accounting Policies (Details) $ / shares in Units, $ in Thousands | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Dec. 31, 2020 USD ($) $ / shares | Sep. 30, 2020 USD ($) $ / shares | Jun. 30, 2020 USD ($) $ / shares | Mar. 31, 2020 USD ($) $ / shares | Dec. 31, 2019 USD ($) $ / shares | Sep. 30, 2019 USD ($) $ / shares | Jun. 30, 2019 USD ($) $ / shares | Mar. 31, 2019 USD ($) $ / shares | Dec. 31, 2020 USD ($) segment $ / shares | Dec. 31, 2019 USD ($) $ / shares | Dec. 31, 2018 USD ($) $ / shares | Jan. 01, 2020 USD ($) | Dec. 31, 2017 USD ($)
Organization Consolidation and Presentation of Financial Statements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of business segments | segment |  |  |  |  |  |  |  |  | 2 |  |  |  |
Contract retention provision |  |  |  |  |  |  |  |  | 10.00% |  |  |  |
Revenue recognized, related to change orders and claims included as contract price adjustments |  |  |  |  |  |  |  |  | $ 14,700 | $ 35,900 |  |  |
Increase (decrease) in operating income |  |  |  |  |  |  |  |  | 86,545 | 57,178 | $ 50,312 |  |
Increase (decrease) in net income | $ 18,150 | $ 17,292 | $ 13,385 | $ 9,932 | $ 12,775 | $ 10,355 | $ 7,207 | $ 7,353 | $ 58,759 | $ 37,690 | $ 31,087 |  |
Increase (decrease) in diluted earnings per common share (in dollars per share) | $ / shares | $ 1.07 | $ 1.02 | $ 0.80 | $ 0.59 | $ 0.76 | $ 0.62 | $ 0.43 | $ 0.44 | $ 3.48 | $ 2.26 | $ 1.87 |  |
Advertising cost |  |  |  |  |  |  |  |  | $ 700 | $ 800 | $ 700 |  |
Allowance for contract retainage | $ 400 |  |  |  | $ 100 |  |  |  | $ 400 | 100 |  |  |
Option to extend lease term (up to) |  |  |  |  |  |  |  |  | 5 years |  |  |  |
Option to terminate leases, period |  |  |  |  |  |  |  |  | 1 year |  |  |  |
Decrease in retained earnings | 429,292 |  |  |  | 364,475 |  |  |  | $ 429,292 | 364,475 | 324,464 |  | $ 287,039
Allowance for contract assets | 359 |  |  |  | 147 |  |  |  | 359 | 147 |  | $ 400 |
Retained Earnings |  |  |  |  |  |  |  |  |  |  |  |  |
Organization Consolidation and Presentation of Financial Statements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Decrease in retained earnings | 270,480 |  |  |  | 212,219 |  |  |  | $ 270,480 | 212,219 | $ 174,736 |  | 143,241
Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |  |  |  |  |  |  |  |
Organization Consolidation and Presentation of Financial Statements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Decrease in retained earnings |  |  |  |  | (268) |  |  |  |  | (268) |  |  | 695
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings |  |  |  |  |  |  |  |  |  |  |  |  |
Organization Consolidation and Presentation of Financial Statements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Decrease in retained earnings |  |  |  |  | $ (268) |  |  |  |  | $ (268) |  | $ (300) | $ 695
Revenue from Contract with Customer | Top Ten Customers | Customer Concentration Risk |  |  |  |  |  |  |  |  |  |  |  |  |
Organization Consolidation and Presentation of Financial Statements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Concentration risk |  |  |  |  |  |  |  |  | 32.70% | 30.80% | 32.90% |  |
Labor employees covered by collective bargaining agreements | Labor Force Concentration Risk |  |  |  |  |  |  |  |  |  |  |  |  |
Organization Consolidation and Presentation of Financial Statements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Concentration risk |  |  |  |  |  |  |  |  | 88.00% |  |  |  |
Contingencies Excluding Wildfire and Health Insurance |  |  |  |  |  |  |  |  |  |  |  |  |
Organization Consolidation and Presentation of Financial Statements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance coverage deductible | 1,000 |  |  |  |  |  |  |  | $ 1,000 |  |  |  |
Wildfire |  |  |  |  |  |  |  |  |  |  |  |  |
Organization Consolidation and Presentation of Financial Statements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance coverage deductible | 2,000 |  |  |  |  |  |  |  | 2,000 |  |  |  |
Health Insurance |  |  |  |  |  |  |  |  |  |  |  |  |
Organization Consolidation and Presentation of Financial Statements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance coverage deductible | $ 200 |  |  |  |  |  |  |  | $ 200 |  |  |  |
Minimum |  |  |  |  |  |  |  |  |  |  |  |  |
Organization Consolidation and Presentation of Financial Statements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Remaining lease term |  |  |  |  |  |  |  |  | 1 year |  |  |  |
Maximum |  |  |  |  |  |  |  |  |  |  |  |  |
Organization Consolidation and Presentation of Financial Statements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Remaining lease term |  |  |  |  |  |  |  |  | 6 years |  |  |  |
Contracts Accounted for under Percentage of Completion |  |  |  |  |  |  |  |  |  |  |  |  |
Organization Consolidation and Presentation of Financial Statements [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in consolidated gross margin |  |  |  |  |  |  |  |  | (0.80%) | (0.80%) | (0.70%) |  |
Increase (decrease) in operating income |  |  |  |  |  |  |  |  | $ (18,000) | $ (11,700) | $ (10,500) |  |
Increase (decrease) in net income |  |  |  |  |  |  |  |  | $ (12,800) | $ (7,500) | $ (8,200) |  |
Increase (decrease) in diluted earnings per common share (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  | $ (0.76) | $ (0.45) | $ (0.49) |  |

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