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Summary of Significant Accounting Policies (Details 3) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 12 Months Ended
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Dec. 31, 2012 | Sep. 30, 2012 | Jun. 30, 2012 | Mar. 31, 2012 | Dec. 31, 2011 | Sep. 30, 2011 | Jun. 30, 2011 | Mar. 31, 2011 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Customer concentration |  |  |  |  |  |  |  |  |  |  |
Revenues | $ 480,883 | $ 431,842 | $ 337,436 | $ 291,573 | $ 373,066 | $ 375,483 | $ 351,956 | $ 359,645 | $ 1,541,734 | $ 1,460,150 | $ 941,765
Revenues | Customer concentration |  |  |  |  |  |  |  |  |  |  |
Customer concentration |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 690,670 | 679,787 | 389,040
Percentage of concentration risk |  |  |  |  |  |  |  |  | 44.80% | 46.50% | 41.20%
Revenues | Customer concentration | Top ten customers |  |  |  |  |  |  |  |  |  |  |
Customer concentration |  |  |  |  |  |  |  |  |  |  |
Number of top customers |  |  |  |  |  |  |  |  | 10 |  |
Number of calendar years in which top customers typically generate minimum specified percentage of revenue |  |  |  |  |  |  |  |  | 1 |  |
Revenues |  |  |  |  |  |  |  |  | 862,300 | 1,000,000 |
Percentage of concentration risk |  |  |  |  |  |  |  |  | 55.90% | 68.50% | 55.30%
Revenues | Customer concentration | Top ten customers | Minimum |  |  |  |  |  |  |  |  |  |  |
Customer concentration |  |  |  |  |  |  |  |  |  |  |
Minimum percentage of revenues generated by top ten customers |  |  |  |  |  |  |  |  | 50.00% |  |
Revenues | Customer concentration | Louisiana DOT |  |  |  |  |  |  |  |  |  |  |
Customer concentration |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 170,899 | 239,516 | 193,139
Percentage of concentration risk |  |  |  |  |  |  |  |  | 11.10% | 16.40% | 20.50%
Revenues | Customer concentration | Large gas and electric utility |  |  |  |  |  |  |  |  |  |  |
Customer concentration |  |  |  |  |  |  |  |  |  |  |
Percentage of concentration risk |  |  |  |  |  |  |  |  | 14.60% | 11.30% |
Revenues | Customer concentration | Ruby contract |  |  |  |  |  |  |  |  |  |  |
Customer concentration |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | $ 12,600 | $ 274,900 |
Percentage of concentration risk |  |  |  |  |  |  |  |  | 0.80% | 18.80% |
Revenues | Customer concentration | One customer |  |  |  |  |  |  |  |  |  |  |
Customer concentration |  |  |  |  |  |  |  |  |  |  |
Percentage of concentration risk |  |  |  |  |  |  |  |  | 14.60% | 11.30% |
Accounts receivable | Customer concentration | One customer |  |  |  |  |  |  |  |  |  |  |
Customer concentration |  |  |  |  |  |  |  |  |  |  |
Percentage of concentration risk |  |  |  |  |  |  |  |  | 10.00% | 15.50% |
Number of customers |  |  |  |  |  |  |  |  | 1 | 1 |

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