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Employee Retirement Plans - Pension expense (Details) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 USD ($) item | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Employee Retirement Plans |  |  |
Number of regional multi-employer pension plans in which the entity participates | item | 37 |  |
Defined contribution plans | $ 7,595 | $ 4,089 | $ 3,950
Multi-employer plans | 11,224 | 8,677 | 6,593
Total expense related to participation in retirement plans | $ 18,819 | $ 12,766 | $ 10,543

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- Definition Amount of expense related to multiemployer retirement plans. + References No definition available. + Details
Name: | bld_MultiemployerRetirementPlanExpense
Namespace Prefix: | bld_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Represents the number of regional multi-employer pension plans in which the entity participates. + References No definition available. + Details
Name: | bld_NumberOfRegionalMultiemployerPensionPlansInWhichEntityParticipates
Namespace Prefix: | bld_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition The amount of expense related to Company's participation in retirement plans. + References No definition available. + Details
Name: | bld_RetirementPlansExpense
Namespace Prefix: | bld_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_CompensationAndRetirementDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of cost for defined contribution plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 + Details
Name: | us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration