Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation Our consolidated financial statements include the accounts of Forestar Group Inc., all subsidiaries, ventures and other entities in which we have a controlling interest. We account for our investment in other entities in which we have significant influence over operations and financial policies using the equity method (we recognize our share of the entities’ income or loss and any preferential returns and treat distributions as a reduction of our investment). We eliminate all material intercompany accounts and transactions. Noncontrolling interests in consolidated pass-through entities are recognized before income taxes. We prepare our financial statements in accordance with generally accepted accounting principles in the United States, which require us to make estimates and assumptions about future events. Actual results can, and probably will, differ from those we currently estimate. At year-end 2016, we had divested substantially all of our oil and gas working interest properties. As a result of this significant change in our operations, we have reported the results of operations and financial position of these assets as discontinued operations within the consolidated statements of income (loss) and consolidated balance sheets for all periods presented. In addition, in 2016, we changed the name of the oil and gas segment to mineral resources to reflect the strategic shift from oil and gas working interest investments to owned mineral interests.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents include cash and other short-term instruments with original maturities of three months or less.
Cash Flows	Cash Flows The consolidated statements of cash flows for 2017, 2016 and 2015 reflect cash flows from both continuing and discontinued operations. Expenditures for the acquisition and development of single-family and multifamily real estate that we intend to develop for sale are classified as operating activities. Expenditures for the acquisition and development of properties to be held and operated, investment in oil and gas properties and equipment, and business acquisitions are classified as investing activities.
Environmental and Asset Retirement Obligations	Environmental and Asset Retirement Obligations We recognize environmental remediation liabilities on an undiscounted basis when environmental assessments or remediation are probable and we can reasonably estimate the cost. We adjust these liabilities as further information is obtained or circumstances change. With the sale of our remaining oil and gas entities in 2017 we no longer have asset retirement obligations related to the abandonment and site restoration requirements that result from the acquisition, construction and development of oil and gas working interest properties, which we have divested. Prior to the sale, we recorded the fair value of a liability for an asset retirement obligation in the period in which it was incurred and a corresponding increase in the carrying amount of the related long-lived asset. Accretion expense related to the asset retirement obligation and depletion expense related to capitalized asset retirement costs are included in cost of mineral resources and in discontinued operations on our consolidated statements of income (loss).
Fair Value Measurements	Fair Value Measurements Financial instruments for which we did not elect the fair value option include cash and cash equivalents, accounts and notes receivables, other assets, debt, accounts payable and other liabilities. With the exception of long-term notes receivable and debt, the carrying amounts of these financial instruments approximate their fair values due to their short-term nature.
Goodwill and Other Intangible Assets	Goodwill and Other Intangible Assets We record goodwill when the purchase price of a business acquisition exceeds the estimated fair value of net identified tangible and intangible assets acquired. We do not amortize goodwill or other indefinite lived intangible assets. Instead, we measure these assets for impairment based on the estimated fair values at least annually or more frequently if impairment indicators exist. We perform the annual impairment measurement in the fourth quarter of each year. Intangible assets with finite useful lives are amortized over their estimated useful lives. In 2017, we sold our remaining owned mineral assets for approximately $85,700,000 and as a result of this sale we recorded a non-cash impairment charge of $37,900,000 related to the mineral resources reporting unit goodwill which is included in cost of mineral resources on our consolidated statements of income (loss). At year-end 2016, we performed our annual goodwill impairment evaluation and concluded that goodwill related to our central Texas water assets was impaired because the carrying value exceeded the fair value and recorded a $3,874,000 non-cash impairment charge
Income Taxes	Income Taxes We provide deferred income taxes using current tax rates for temporary differences between the financial accounting carrying value of assets and liabilities and their tax accounting carrying values. We recognize and value income tax exposures for the various taxing jurisdictions where we operate based on laws, elections, commonly accepted tax positions, and management estimates. We include tax penalties and interest in income tax expense. We provide a valuation allowance for any deferred tax asset that is not likely to be recoverable in future periods. When we believe a tax position is supportable but the outcome uncertain, we include the item in our tax return but do not recognize the related benefit in our provision for taxes. Instead, we record a reserve for unrecognized tax benefits, which represents our expectation of the most likely outcome considering the technical merits and specific facts of the position. Changes to liabilities are only made when an event occurs that changes the most likely outcome, such as settlement with the relevant tax authority, expiration of statutes of limitations, changes in tax law, or recent court rulings.
Property and Equipment	Property and Equipment We carry property and equipment at cost less accumulated depreciation. We capitalize the cost of significant additions and improvements, and we expense the cost of repairs and maintenance. We capitalize interest costs incurred on major construction projects.
Real Estate	Real Estate We carry real estate at the lower of cost or fair value less cost to sell. We capitalize interest costs once development begins, and we continue to capitalize throughout the development period. We also capitalize infrastructure, improvements, amenities, and other development costs incurred during the development period. We determine the cost of real estate sold using the relative sales value method. When we sell real estate from projects that are not finished, we include in the cost of real estate sold estimates of future development costs through completion, allocated based on relative sales values. These estimates of future development costs are reevaluated at least annually, with any adjustments being allocated prospectively to the remaining units available for sale. We receive cash deposits from home builders for purchases of vacant developed lots from community development projects. These earnest money deposits are released to the home builders as lots are developed and sold. In certain instances earnest money deposits are subject to mortgages which are secured by the real estate under contract with the home builder. These mortgages expire when the earnest money is released to the home builders as lots are developed and sold. At year-end 2017, $40,408,000 of real estate was subject to earnest money mortgages, including $25,712,000 classified as assets held for sale. We have agreements with utility or improvement districts, principally in Texas, whereby we agree to convey to the districts water, sewer and other infrastructure-related assets we have constructed in connection with projects within their jurisdiction. The reimbursement for these assets ranges from 70 to 90 percent of allowable cost as defined by the district. The transfer is consummated and we receive payment when the districts have a sufficient tax base to support funding of their bonds. The cost we incur in constructing these assets is included in capitalized development costs, and upon collection, we remove the assets from capitalized development costs. We provide an allowance to reflect our past experiences in collecting these reimbursements. Impairment of Real Estate Long-Lived Assets We review real estate long-lived assets held for use for impairment when events or circumstances indicate that their carrying value may not be recoverable. Impairment exists if the carrying amount of the long-lived asset is not recoverable from the undiscounted cash flows expected from its use and eventual disposition. We determine the amount of the impairment loss by comparing the carrying value of the long-lived asset to its estimated fair value. We generally determine fair value based on the present value of future cash flows expected from the sale of the long-lived asset. Non-cash impairment charges related to our owned and consolidated real estate assets are included in cost of real estate sales and other.
Reclassifications	Reclassifications In 2017, we have reclassified prior years' restricted cash that was included in other assets to a separate line item on our consolidated balance sheets to conform to the current year presentation.
Revenue	Revenue We recognize revenue from sales of real estate when a sale is consummated, the buyer’s initial investment is adequate, any receivables are probable of collection, the usual risks and rewards of ownership have been transferred to the buyer, and we do not have significant continuing involvement with the real estate sold. If we determine that the earnings process is not complete, we defer recognition of any gain until earned. We exclude from revenue amounts we collect from utility or improvement districts related to the conveyance of water, sewer and other infrastructure related assets. We exclude from revenue amounts we collect from customers that represent sales tax or other taxes that are based on the sale. These amounts are included in other accrued expenses until paid.
Share-Based Compensation	Share-Based Compensation We use the Black-Scholes option pricing model to determine the fair value of stock options, and a Monte Carlo simulation pricing model to determine the fair value of market-leveraged stock units and for stock options with market conditions. The fair value of equity-settled awards is determined on the grant date and the fair value of cash-settled awards is determined at period end. We expense share-based awards ratably over the vesting period or earlier based on retirement eligibility.
Owned Mineral Interests	Owned Mineral Interests Historically, we leased our mineral interests to third-party exploration and production entities, we retained a royalty interest and may have taken an additional participation in production, including a working interest. In first quarter 2017, we sold our remaining owned mineral assets.
Oil and Gas Properties (Discontinued Operations)	Oil and Gas Properties (Discontinued Operations) We used the successful efforts method of accounting for our oil and gas producing activities. Costs to acquire mineral interests leased, costs to drill and complete development of oil and gas wells and related asset retirement costs were capitalized. Costs to drill exploratory wells were capitalized pending determination of whether the wells had proved reserves and if determined incapable of producing commercial quantities of oil and gas these costs were expensed as dry hole costs. At year-end 2017, we had no capitalized exploratory well costs pending determination of proved reserves. Exploration costs include dry hole costs, geological and geophysical costs, expired unproved leasehold costs and seismic studies, and were expensed as incurred. Production costs incurred to maintain wells and related equipment were charged to expense as incurred. Depreciation and depletion of producing oil and gas properties was calculated using the units-of-production method. Proved developed reserves were used to compute unit rates for unamortized tangible and intangible drilling and completion costs. Proved reserves were used to compute unit rates for unamortized acquisition of proved leasehold costs. Unit-of-production amortization rates were revised whenever there was an indication of the need for revision but at least once a year and those revisions were accounted for prospectively as changes in accounting estimates. We no longer own any oil and gas working interest properties. Impairment of Oil and Gas Properties (Discontinued Operations) Historically, we evaluated our oil and gas properties, including facilities and equipment, for impairment whenever events or changes in circumstances indicated that the carrying value of the asset may not be recoverable. We estimate the expected undiscounted future cash flows of our oil and gas properties and compared such undiscounted future cash flows to the carrying amount of the oil and gas properties to determine if the carrying amount was recoverable. If the carrying amount exceeded the estimated undiscounted future cash flows, we adjusted the carrying amount of the oil and gas properties to fair value. The factors used to determine fair value were subject to our judgment and expertise and included, but were not limited to, recent sales prices of comparable properties, the present value of future cash flows net of estimated operating and development costs using estimates of proved reserves, future commodity pricing, future production estimates, anticipated capital expenditures, and various discount rates commensurate with the risk and current market conditions associated with realizing the expected cash flows projected. Assessing unproved leasehold properties to determine impairment required significant judgment. We assessed our unproved leasehold properties periodically for impairment on a property-by-property basis based on remaining lease terms, drilling results or future plans to develop acreage. Impairment expense for proved and unproved oil and gas properties are included in cost of mineral resources and cost of oil and gas producing activities in discontinued operations.
New and Pending Accounting Pronouncements	New and Pending Accounting Pronouncements Adoption of New Accounting Standards In March 2016, the FASB issued ASU 2016-09, Compensation-Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting, as part of its simplification initiative. The areas for simplification in this update involve several aspects of the accounting for share-based payment transactions, including income tax consequences, classification of awards as either equity or liabilities, and the classification on the statement of cash flows. We adopted the updated standard on January 1, 2017. Effective first quarter 2017, stock-based compensation (SBC) excess tax benefits or deficiencies are reflected in the consolidated statements of income (loss) as a component of the provision for income taxes, whereas they previously were recognized in equity to the extent additional paid-in capital pool was available. Additionally, our consolidated statements of cash flows will now present excess tax benefits as an operating activity, if applicable. Finally, we have elected to account for forfeitures as they occur, rather than estimate expected forfeitures. The adoption of this guidance did not have a material impact on our consolidated financial statements. Pending Accounting Standards In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606), requiring an entity to recognize the amount of revenue to which it expects to be entitled for the transfer of promised goods or services to customers. The updated standard will replace most existing revenue recognition guidance in U.S. GAAP when it becomes effective and permits the use of either the retrospective or cumulative effect transition method. The updated standard becomes effective for annual and interim periods beginning after December 15, 2017. Due to our change in fiscal year-end, this standard is effective for us beginning October 1, 2018. The guidance permits two methods of adoption: retrospectively to each prior reporting period presented (full retrospective method), or retrospectively with the cumulative effect initially applying the guidance recognized at the date of initial application (the cumulative catch-up transition method). We currently anticipate adopting the standard using the cumulative catch-up transition method. We anticipate this standard will not have a material impact on our consolidated financial statements. While we are continuing to assess all potential impacts of the standard, we expect revenue related to lot and tract sales to remain substantially unchanged. Due to the complexity of certain of our real estate sale transactions, the revenue recognition treatment required under the standard will be dependent on contract-specific terms, and may vary in some instances from recognition at the time of the sale closing. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). This ASU requires lessees to put most leases on their balance sheets but recognize expenses on their income statements in a manner that is similar to today's accounting. This guidance also eliminates today's real estate-specific provisions for all entities. For lessors, the guidance modifies the classification criteria and the accounting for sales-type and direct financing leases. This guidance is effective in 2019, and interim periods within that year. Early adoption is permitted. The new leases standard requires a modified retrospective transition approach for all leases existing at, or entered into after, the date of initial application, with an option to use certain transition relief. We are currently evaluating the effect the updated standard will have on our financial position and disclosures. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230), in order to address eight specific cash flow issues with the objective of reducing the existing diversity in practice. The updated standard is effective for financial statements issued for annual periods beginning after December 15, 2017 and interim periods within those fiscal years with early adoption permitted. We are currently evaluating the effect that the updated standard will have on our earnings, financial position and disclosures, but we do not expect it to have a material effect on our consolidated financial statements. In November, 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230). This ASU requires that a statement of cash flow explain the change during the period in the total of cash, cash equivalents, and amounts generally described as restricted cash or restricted cash investments. This standard is effective for fiscal years beginning after December 15, 2017. The adoption of ASU 2016-18 will modify our current disclosures and reclassifications relating to the consolidated statements of cash flows, but we do not expect it to have a material effect on our consolidated financial statements. In May 2017, the FASB issued ASU 2017-09, Compensation - Stock Compensation (Topic 718), in order to provide guidance about which changes to terms or conditions of a share-based payment award require an entity to apply modification accounting in Topic 718. The updated standard is effective for financial statements issued for annual periods beginning after December 15, 2017. We are currently evaluating the effect that the updated standard will have on our earnings, financial position and disclosures, but we do not expect it to have a material effect on our consolidated financial statements.