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Employee Benefit Plans (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
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Dec. 31, 2013
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Dec. 31, 2012
Employee Benefit Plans      
Percentage of contribution of covered employee's salaries or wages 2.50%fix_DefinedContributionPlanMaximumAnnualContributionPerEmployeeByEmployerPercent    
Contribution $ 6,100,000us-gaap_DefinedContributionPlanCostRecognized $ 5,500,000us-gaap_DefinedContributionPlanCostRecognized $ 5,200,000us-gaap_DefinedContributionPlanCostRecognized
Amount payable to plan 100,000fix_DefinedContributionPlanAmountPayable 100,000fix_DefinedContributionPlanAmountPayable  
Number of employees who are union members 6fix_MultiEmployerPensionPlanNumberOfEmployeesWhoAreUnionMembers 6fix_MultiEmployerPensionPlanNumberOfEmployeesWhoAreUnionMembers  
Contributions made to multi employer pension plans 0us-gaap_MultiemployerPlansPlanContributions 0us-gaap_MultiemployerPlansPlanContributions 0us-gaap_MultiemployerPlansPlanContributions
Period in which certain individuals are entitled to fixed annual payments 15 years    
Maximum age under which certain individuals are entitled to fixed annual payments 65 years    
Portion of benefits vesting after ten years of completed service (as a percent) 50.00%fix_DefinedBenefitPlanPeriodPortionOfBenefitsVestingAfterTenYearsOfCompletedService    
Period of completed service over which 50% of benefits are vested 10 years    
Portion of benefits vesting after fifteen years of completed service (as a percent) 75.00%fix_DefinedBenefitPlanPeriodPortionOfBenefitsVestingAfterFifteenYearsOfCompletedService    
Period of completed service over which 75% of benefits are vested 15 years    
Period of service over which benefits are fully vested 20 years    
Unfunded benefit liability $ 3,000,000us-gaap_DefinedBenefitPlanFundedStatusOfPlan $ 3,200,000us-gaap_DefinedBenefitPlanFundedStatusOfPlan