Commitments and Contingencies (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013 item	Dec. 31, 2012 item
Commitments and Contingencies
Number of irrevocable standby letters of credit outstanding	2	2
Number of letters of credit related to the payment obligation under insurance programs	1	1
Letters of credit outstanding related to the payment obligation under insurance programs	$ 17.5	$ 17.5
Letters of credit outstanding related to contract performance obligations	2.2	2.2
Future minimum lease payments for operating leases
Remainder of 2013	2.0
2014	1.2
2015	0.5
2016	0.2
2017	0.2
Thereafter	0.4
Purchase Commitments for Construction Equipment
Outstanding purchase orders for certain construction equipment	$ 7.5
Scheduled period of time for cash outlay requirements	6 months
Minimum
Leases
Term of operating lease	1 year
Maximum
Leases
Term of operating lease	9 years

Represents the amount of the letters of credit related to contract performance obligations.

No definition available.

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Balance Type:	credit
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- Definition

Represents the letters of credit outstanding related to the payment obligation under insurance programs.

+ References

No definition available.

+ Details

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- Definition

Represents the number of irrevocable standby letters of credit outstanding.

+ References

No definition available.

+ Details

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- Definition

Represents the number of letters of credit related to the payment obligation under insurance programs.

+ References

No definition available.

+ Details

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- Definition

Represents the term of the operating lease arrangements of the entity.

+ References

No definition available.

+ Details

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- Details

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- Definition

Represents the schedule period of time for cash outlay requirements under an existing purchase commitment.

+ References

No definition available.

+ Details

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- Definition

Amount of required minimum rental payments maturing in the fifth fiscal year following the latest fiscal year for operating leases having an initial or remaining non-cancelable letter-terms in excess of one year.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 13

-Paragraph 16

-Subparagraph b(i)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 13

-Paragraph 122

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 10

-Section 55

-Paragraph 40

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-Section 50

-Paragraph 2

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- Definition

Amount of required minimum rental payments maturing in the forth fiscal year following the latest fiscal year for operating leases having an initial or remaining non-cancelable letter-terms in excess of one year.

+ References

+ Details

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- Definition

Amount of required minimum rental payments maturing in the third fiscal year following the latest fiscal year for operating leases having an initial or remaining non-cancelable letter-terms in excess of one year.

+ References

+ Details

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- Definition

Amount of required minimum rental payments maturing in the second fiscal year following the latest fiscal year for operating leases having an initial or remaining non-cancelable letter-terms in excess of one year.

+ References

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- Definition

Amount of required minimum rental payments maturing after the fifth fiscal year following the latest fiscal year for operating leases having an initial or remaining non-cancelable letter-terms in excess of one year.

+ References

+ Details

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- Definition

Amount of required minimum rental payments maturing in the remainder of the fiscal year following the latest fiscal year ended for operating leases having an initial or remaining non-cancelable letter-terms in excess of one year.

+ References

No definition available.

+ Details

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- Definition

The floor amount as of the balance sheet date that the entity must expend to satisfy the terms of disclosed arrangements (excluding long-term commitments) in which the entity has agreed to expend funds to procure goods or services from one or more suppliers, other than under a long-term purchase commitment or an unconditional purchase obligation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 17

-Article 9

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-Publisher SEC

-Name Regulation S-X (SX)

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