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Company Retirement Plans (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
401(k) Plan |  |  |
Company retirement plans |  |  |
Maximum contribution by employees (as a percent) | 100.00% |  |
Employer match of employee contributions on first level of eligible compensation (as a percent) | 100.00% |  |
Percentage of eligible compensation, first level, matched by employer | 3.00% |  |
Employer match of employee contributions on the second level of eligible compensation (as a percent) | 50.00% |  |
Percentage of eligible compensation, second level, partially matched by employer | 2.00% |  |
Employer's contribution | $ 2,267 | $ 1,963 | $ 1,734
Born Heaters Canada, ULC RRSP-DPSP Plan |  |  |
Company retirement plans |  |  |
Employer match of employee contributions on first level of eligible compensation (as a percent) | 100.00% |  |
Percentage of eligible compensation, first level, matched by employer | 3.00% |  |
Employer match of employee contributions on the second level of eligible compensation (as a percent) | 50.00% |  |
Percentage of eligible compensation, second level, partially matched by employer | 2.00% |  |
Employer's contribution | $ 69 | $ 78 | $ 86
Number of components of the plan | 2 |  |
Number of years of employment as a vesting period of DPSP portion | 1 year |  |

X
- Definition Represents the number of years of employment, considered as a vesting period of the DPSP Portion of the Registered Retirement Saving Plan - Deferred Profit Sharing Plan. + References No definition available. + Details
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Balance Type: | na
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X
- Definition Represents the second level of employee contributions (percentage of compensation) which are partially matched by the employer. + References No definition available. + Details
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X
- Definition Represents the employer matching contribution of the first level of employee contributions. + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

X
- Definition Represents the employer matching contribution of the second level of employee contributions. + References No definition available. + Details
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Period Type: | duration

X
- Definition Represents the first level of employee contributions (percentage of compensation) which are matched by the employer. + References No definition available. + Details
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Namespace Prefix: | prim_
Data Type: | num:percentItemType
Balance Type: | na
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X
- Details
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X
- Definition Represents the maximum contribution by eligible employees, as a percentage of their salary, subject to the annual limit prescribed by Internal Revenue Service. + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

X
- Definition Represents the number of components of the defined contribution plan. + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

X
- Definition The amount of the cost recognized during the period for defined contribution plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 11 -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. + Details
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