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Derivatives - Quantitative Information About Level 3 Fair Value Measurements (Detail) (USD $) In Thousands, unless otherwise specified |  |  | 12 Months Ended
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Sep. 30, 2013 Series F Agreement [Member] | Dec. 31, 2012 Series F Agreement [Member] | Dec. 31, 2012 Discounted Cash Flow [Member] Unobservable Inputs (Level 3) Derivative Financial Instruments, Liabilities [Member] Minimum | Dec. 31, 2012 Discounted Cash Flow [Member] Unobservable Inputs (Level 3) Derivative Financial Instruments, Liabilities [Member] Maximum
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] |  |  |  |  |  |
Fair value inputs, long dated treasuries |  |  | 2.82% | [1] | 2.91% | [1]
Fair value inputs, own credit risk |  |  | 0.98% | [2] | 1.59% | [2]
Fair Value | $ 0 | $ (826) |  |  |  |
[1] | (B)Represents the forward 30 year Treasury CMT Rate.
[2] | (C)Represents credit default swap spread curve used in the valuation analysis at December 31, 2012.

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- Definition Fair value inputs long dated treasuries. + References No definition available. + Details
Name: | fr_FairValueInputsLongDatedTreasuries
Namespace Prefix: | fr_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Risk associated with risk of default, credit spread risk and downgrade risk, expressed as a percentage, used as an input to measure fair value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
Name: | us-gaap_FairValueInputsEntityCreditRisk
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_FairValueInputsLiabilitiesQuantitativeInformationLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition This element represents the aggregate of the liabilities reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
Name: | us-gaap_LiabilitiesFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant