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Debt (Details Textual) $ in Millions | 1 Months Ended | 6 Months Ended | 9 Months Ended |
--- | --- | --- | --- | ---
Jun. 30, 2016 USD ($) | Jun. 30, 2016 USD ($) | Sep. 30, 2018 USD ($) | Dec. 31, 2017 USD ($)
Debt Instrument [Line Items] |  |  |  |
Letters of Credit Outstanding, Amount |  |  | $ 21.2 | $ 20.9
Debt Issuance Costs, Line of Credit Arrangements, Net |  |  | $ 0.6 |
Debt Instrument Covenant Leveraged Debt Ratio Restriction |  |  | 2.25 |
Insurance Program Obligations [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Letters of Credit Outstanding, Amount |  |  | $ 17.6 | 17.6
Contract Performance Obligations [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Letters of Credit Outstanding, Amount |  |  | $ 3.6 | $ 3.3
Credit Facility Amendment [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Letters of Credit Outstanding, Amount | $ 150.0 | $ 150.0 |  |
Line of Credit Facility, Initiation Date | Sep. 28, 2018 |  |  |
Maximum [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Leverage Coverage Ratio Expansion |  |  | 3.5 |
Leverage Coverage Ratio |  |  | 3.0 |
Maximum [Member] | Revolving Credit Facility [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage |  |  | 0.375% |
Maximum [Member] | Performance letters of credit [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Letter of credit fee (as a percent) |  |  | 1.125% |
Maximum [Member] | Non-performance Letters Of Credit [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Interest rate margin (as a percent) |  |  | 2.125% |
Maximum [Member] | London Interbank Offered Rate (LIBOR) [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Interest rate margin (as a percent) |  |  | 2.00% |
Maximum [Member] | Base Rate [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Interest rate margin (as a percent) |  |  | 1.00% |
Minimum [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Interest coverage ratio |  |  | 3.0 |
Minimum [Member] | Revolving Credit Facility [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage |  |  | 0.20% |
Minimum [Member] | Performance letters of credit [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Letter of credit fee (as a percent) |  |  | 0.625% |
Minimum [Member] | Non-performance Letters Of Credit [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Interest rate margin (as a percent) |  |  | 1.125% |
Minimum [Member] | London Interbank Offered Rate (LIBOR) [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Interest rate margin (as a percent) |  |  | 1.00% |
Minimum [Member] | Base Rate [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Interest rate margin (as a percent) |  |  | 0.00% |
Syndicated Credit Agreement [Member] | Facility [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Option to increase borrowing capacity |  | 100.0 |  |
Maximum borrowing capacity | $ 250.0 | $ 250.0 |  |
Credit Agreement [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt, Weighted Average Interest Rate |  |  | 2.90% |
Credit Agreement [Member] | Facility [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Percentage of Capital Stock From Direct Foreign Subsidiaries |  | 65.00% |  |
Maximum Acquisition Consideration Under Credit Agreement |  |  | $ 50.0 |
Credit Agreement [Member] | Foreign Revolving Loans and Letters of Credit [Member] | Facility [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Maximum borrowing capacity | $ 50.0 | $ 50.0 |  |

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- Definition The Company has the option to expand the debt coverage ratio to 3.5 in the event of an acquisition in excess of $30 million. The expansion occurs in the quarter the acquisition is consummated and shall continue in effect for the next four consecutive quarters. + References No definition available. + Details
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